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  • 1985-1989  (324)
  • 1986  (324)
  • Atomic, Molecular and Optical Physics  (324)
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 185-189 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: This paper presents a survey of published and unpublished ab initio calculations of the vibrational structures of the ten lowest electronic singlet states of the hydrogen molecule up to the H(n = 1) + H(n = 2) dissociation limit. The data are based on adiabatic potential functions (clamped-nuclei electronic energies and nuclear-mass-dependent diagonal corrections). Nonadiabatic coupling has been treated ab initio within the five states. of 1Λg+ symmetry (X,EF, GK, HH̄) and 1Σ+g I.1Πg. The accuracies of the theoretical energies are determined by comparisons with experimental data for H2, HD, and D2. The level shifts and predissociation probabilities of the excited 1Σ+g states, generated by nonadiabatic coupling with the discrete and continuous vibrational structure of the ground state, and radiative properties have also been calculated.
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 311-314 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The general problem of relativistic corrections to the kinetic energy in quasirelativistic theories, is discussed and related formulas are developed. It is shown that the well-known mass-velocity operator, Hmv = (-α2/8)p4, is incorrect and does not provide any proper relativistic corrections.
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 333-343 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A simple convexity relation for the nuclear charges of isoelectronic molecules implies some energy relations and topological constraints on their respective reaction globes. These energy relations take the form of a “rule on nested reaction globes.” These reaction globes and their special cases of conformational globes describe the interrelations within families of chemical reactions and conformational changes, respectively. The nuclear charge relations lead to constraints on the relative positioning of reaction globes. These relative positions provide a concise description of energy relations for families of reactions. Conditions are given for a special case, when one reaction globe is completely enclosed by another. The general conclusions are illustrated by results of ab initio potential surface calculations on the conformational problem of 1,4-disubstituted 2-butenes.
    Additional Material: 5 Ill.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 323-332 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetic-energy anisotropies of fifteen diatomic hydrides AH with A = H, Li, Be, B, C, N, O, F, Na, Mg, Al, Si, P, S, Cl are calculated from self-consistent-field wave functions constructed from extended basis sets of Slater-type orbitals. It is found that there is no consistent ordering of the bond-parallel and bond-perpendicular components of the kinetic energy with respect to separated atom values. An analysis of the orbital contributions reveals that nonbonding π orbitals make large contributions to the total kinetic-energy anisotropy. This makes it difficult, if not impossible, to deduce anything about the nature of the chemical bond from the total anisotropy. However, certain dimensionless orbital kinetic-energy anisotropies are useful for interpretative studies because, in free atoms, these quantities have fixed values that depend only on the symmetry of the orbital.
    Additional Material: 4 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 351-360 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The minimal energy conformations of o-benzosemiquinone anion radicals in the solvents containing the alkali metals and the alkali earth metals were calculated by using the INDO method and the molecular geometry adjusting method. Our previous calculations of o-benzosemiquinone anion radical in the lithium ion solution showed that the total energy of the minimal energy conformation in the case of the assignment |A3| 〉 |A4| were lower by approximately 320 kcal/mol than that in the case of the assignment |A3| 〉 |A4|. Therefore, the calculations of the minimal energy conformations of o-benzosemiquinone anion radicals in the metal ion solutions were carried out in the cases of the assignment |A3| 〉 |A4|. The conformations in both assignments A3 〈 A4 〈 0 and (A3 〈 0 〈 A4 and |A3| 〉 |A4|) were calculated.Although the metal ions were not explicitly contained in the molecular geometries, the minimal energy conformations are considered to be close to the real minimal energy conformations of o-benzosemiquinone anion radicals in the metal ion solutions. The C—O bond lengths has a good positive rank correlation with the stabilities of o-benzosemiquinone radicals in the divalent metal ion solutions and the bond angles ∠C3C4H4 had a good negative rank correlation with those stabilities.
    Additional Material: 3 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 361-371 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The minimal energy conformations of o-benzosemiquinone anion radical were calculated for several cases of assignments by using the INDO method and the molecular geometry adjusting method. In order to know the effect of lithium ion in the solvent, the minimal energy conformations of the system of Li—O—H and o-benzosemiquinone anion radical were calculated. The calculations of the minimal energy conformations of this radical in t-butyl alcohol, alkaline aqueous ethanol, alkaline water, neutral methanol, and acetonitrile were carried out. The total energies of the minimal energy conformations in the assignment |A3| 〉 |A4| were lower than those in the assignment |A3| 〈 |A4|.
    Additional Material: 5 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 379-391 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The almost lossless transfer of energy and electronic charge along quasi-one-dimensional molecular substances such as peptide chains (H — N — C = O)n found an explanation in a model proposed by Davydov. The Davydov model is based on the dipole - dipole interactions between neighboring peptide groups and on the fact that the internal C=O vibrations are coupled to the elastic deformations of the chain. The Davydov Hamiltonian is written in the position-space representation and, on making a continuum transformation, leads to a nonlinear Schrödinger equation whose solutions are solitons. A Davydov soliton is a coupled pair of an exciton and a lattice deformation. In this paper, the Davydov Hamiltonian is transformed to the reciprocal lattice and its equivalent, second-quantized Hamiltonian is investigated. Some important observations are made about the coupling constants, their dispersion relations, and the equation of motion for the ladder operators. Our procedure is free of semiclassical approximations but, instead, assumes the onset of Bose condensation. The resultant nonlinear Schrödinger equation is similar to that of Davydov, but a more complete set of solutions is found.
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  • 8
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 407-424 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The correlation hole of Coulson and Nielson and its extension to momentum space by Banyard and Reed is studied by using an exactly solvable model. For this model all relevant quantities pertaining to the correlation hole have been calculated exactly. We use this model to study the relationship between the fit to the correlation hole for an approximate wave function and the closeness of the approximate energies to the exact ones. We show that, although in general the better the fit the closer are the approximate physical quantities to the exact ones, there are exceptions where that is not the case. Also, we present a convenient method for the calculation of the two particle distribution in momentum space and generalize the concept of the correlation hole by defining it in the pseudophase space of position and momentum.
    Additional Material: 10 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 399-406 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A state-specific approach to the calculation of transition moments between molecular electronic states requires that the wavefunction for each state is expanded in its optimum one-electron basis and that nonorthonormal basis techniques are used for the calculation of the transition moment integrals. A method has been developed for carrying out such nonorthonormal basis calculations, based on the corresponding orbitals transformation and appropriately defined density matrices, which may be used with configuration interaction (CI) wavefunctions. Further improvements of the method have resulted in a decrease in the time required for the calculations and thus allow its application with moderately large CI expansions for each state. Nonorthonormal basis calculations on transition moments in H2O have been carried out using the above method. The results are in agreement with those of large MRD-CI calculations.
    Additional Material: 3 Tab.
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  • 10
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 425-433 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A hermitian, variational open-shell coupled-cluster method is described and applied to the calculation of H2O and N2 ionization potentials in the T ≈ T2 approximation. A nonvariational calculation is also carried out, with the inclusion of T1 and T3 in addition to T2. Both methods give fair agreement with experiment when only T2 is taken into account. T3, which is included at present in the nonvariational scheme only, has a considerable effect on the results and gives good agreement with experiment.
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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 461-468 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe how to select optimal values of the time step τ and how to allocate computer time in order to minimize the error bar of the unbiased energy estimate (τ = 0 limit) in diffusion Monte Carlo simulations (DMC). Several polynomial models are considered, appropriate for both linearly biased and quadratically biased algorithms. An efficient nonoptimal design is also reported, which has the advantage of checking the model's adequacy. Further, we compare the efficiency of linearly biased and quadratically biased DMC, and we estimate the efficiency of DMC versus Green's function Monte Carlo.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 435-460 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The accurate treatment of molecular excited states is based on a matrix solution of the electronic Schrödinger equation in a suitable AO basis which relies heavily on the existence of a good approximate description of a given eigenfunction prior to the solution of a secular equation. The manner in which one or more roots corresponding to excited states can be traced from a rather simple starting point to the final results obtained from a large-scale CI treatment is outlined. In such calculations it is very useful to have a measure of the energy-lowering capability of configurations which differ by at most a double excitation from a series of reference configurations which represent the dominant contributions to one or more electronic states. The efficient computation of the required Hamiltonian matrix elements for this purpose, as well as for the construction of the final secular equations, can be achieved through the use of the table CI method, which does not rely on any special relationship between configurations to complete its work. Employing CI expansions of length no greater than 10000-20000 terms is very advantageous for the calculation of special properties of the resulting wavefunctions, especially those of spin-dependent quantummechanical operators. With the help of perturbative corrections it is possible to obtain energy results with such methods which are accurate to within 0.01 hartree of the full CI limit [R. A. Phillips, R. J. Buenker, P. J. Bruna, and S. D. Peyerimhoff, Chem. Phys. 84, 11 (1984)], whose error is small compared to that inherent in the use of conventional double-zeta-plus-polarization AO basis sets, therefore making further reduction of the CI errors almost inconsequential. Numerous examples for the calculation of the energy and properties of molecular excited states are surveyed.
    Additional Material: 3 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 469-476 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The first absorption peak in the UV spectrum of polyene is interpreted in terms of charge transfer excitons. The exciton spectrum has been calculated from first principles using the Green's function formalism of charge transfer exciton theory. Electron correlation effects on the polyene band structure have been included with the help of second order Møller-Plesset perturbation theory and of the electron polaron model of Toyozawa. The spectrum of bound singlet excitons starts at EK=0 = 1.86 eV above the top of the valence band. A deeper lying triplet level is observed at 0.72 eV. Further correlation effects on the band gap and dielectric screening of the electron-hole ineraction are discussed.
    Additional Material: 2 Ill.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 477-483 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The definition and properties of the bond order and valence indices calculated from ab initio wave functions are summarized. Their physical interpretation relationships to the exchange effects in bonding and generalization to correlated wave functions are also discussed. Some examples with typical bond order and valence values are shown.
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 485-495 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We explore a generalized weak-field model for the description of the electronic properties of a partly filled shell(S) ion in a crystalline field. Such a model corresponds to the one developed, in parts III and IV of this series, for dN and fN ions in cubical symmetry except that the constraint relations are relaxed. This leads to a fourteen-parameter weak-field model for dN ions in octahedral symmetry and to a 33-parameter weak-field model for fN ions in octahedral symmetry. The latter two models are completely equivalent to the corresponding strong-field models as developed by Griffith and by Tanabe, Sugano, and Kamimura. The constraint relations of parts III and IV are further discussed. In particular, the role they play in fitting procedures is examined. As a conclusion, the weak-field model of III and IV appears as a phenomenological version of the generalized weak-field model introduced in this paper.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 511-518 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Defined as the formation of two new bonds of equal length at the same time, synchronism has been shown to be not possible for the Diels-Alder reaction when the products are not of strict Cs symmetry. This has been done in two steps. First, the SCF-MO transition-state structures for the cyclization reactions of ethylene with cyclopentadiene and furan have been calculated. The activation energies (and heat of reaction) with the STO-3G, 3-21G, and 4-31G basis sets are as follows: with cyclopentadiene 35.0 (-80.7), 30.0 (-32.0), 38.4 (-19.9) kcal/mol; with furan 33.4 (-66.4), 34.6 (-16.5), 42.1 (-8.8) kcal/mol. Then, using a bond-order-bond-length relationship, a value for the tendency toward asynchronism has been calculated for substituted reactants where the two new bonds are arbitrarily held equal. This tendency has been shown in all cases which break strict Cs symmetry. Further, inspection of the atomic-orbital coefficients showed that the substituents cannot be considered as mere perturbations on the pi system in many cases.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 497-510 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Frenkel exciton theory is reviewed. This theory yields general formulas relating the absorption and circular dichroism spectra of large molecules or clusters to the optical properties of their subunits. It is shown that for weakly interacting systems, the circular dichroism associated with a band of exciton states is proportional to tr(FH), where F is an “optical matrix” constructed from the positions and transition dipole moments of the subunits, and H is an interaction-energy matrix. It is shown that if a system expnds isotropically, then tr(FH) falls off as the inverse square of the linear dimensions. This result is compared with experimental measurements of the low-temperature optical rotatory dispersion of proteins as a function of temperature. The optical properties of helical polymers are also discussed as a function of their geometrical parameters.
    Additional Material: 8 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 717-735 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A very simple model of DNA in biological media, consisting of electric charges related to the phosphate groups and to counterions immersed in structured media of constant permittivity, is introduced and tested with the study of a model for the first stages of the DNA transcription.This process is modeled into two steps involving three ‘states’: (1) the DNA system at the equilibrium, (2) the DNA with a small portion deprived of counterions, and (3) the DNA with a partial opening of the double helix in correspondence of the zone deprived of counterions.An extensive investigation involving as variable parameters the length of the DNA specimen (from 31 to 1511 base pairs), the amount of condensed counterion charge (from complete compensation to zero), and the geometrical parameters identifying the local opening has shown that the proposed transcription mechanism is reasonable, and that the DNA model considered here may fill a gap between accurate models including all the interactions - and employed at present for small fragments - and unstructured models addressed to inspect the behavior at the limit of infinite DNA length.
    Additional Material: 3 Ill.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 737-753 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Observation of trends in computed spin-orbit splittings for relatively light molecules leads to the conclusion that relativistic corrections to the electronic charge distribution are important when treating molecules containing heavy atoms (Z 〉 18). In order to preserve the nature of the successful computational techniques currently applied to light molecules in so far as possible, particularly to allow for the treatment of correlation effects in an efficient CI procedure on an equal footing with relativistic effects, emphasis is placed on the development of a two-component formalism for this purpose. A first attempt in this direction consists of formulating a spin-free quantum mechanical operator that reflects relativistic kinematics. The mass-velocity term in the Breit-Pauli Hamiltonian is not appropriate for a variational treatment, however, since it drastically alters the spectrum and gives results that are not bounded from below. To avoid this problem the relativistic free particle energy has been used directly for the representation of the kinetic energy, and in addition the Darwin term has been included as a correction to the potential energy. This approach can be justified with reference to the Foldy-Wouthuysen reduction of the Dirac equation, but the class of basis functions used in a variational procedure with this Hamiltonian must be restricted to avoid the formation of a node in the wavefunction at the nucleus; the same problem is circumvented in the Cowan-Griffin method by imposing Dirac boundary conditions on the wavefunction. With this method, accurate spin-orbit splittings have been computed for Br, I, Xe+, CBr, and XeF, but the resulting total energies are found to be overly sensitive to the representation of the inner shells of these systems. Improved results for both valence and inner shells are then shown to follow from the use of the no-pair equation, which provides a variationally tractable two-component method employing a momentum dependent potential that gives a realistic description of relativistic effects for atoms and molecules over a suitably large range of Z.
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  • 20
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 767-777 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The LCAO-X α method is used to compute the X α rotational barrier for ethane both in the rigid-rotor approximation and for optimized staggered and eclipsed geometries. The problem of how to pick points and weights in three dimensions for fitting the nonanalytic exchange and correlation functionals of local density-functional theory is examined; a variational weighting procedure is developed which greatly reduces the size of errors due to the 3-D grid as compared to simple volume-element weighting. The accuracy of the X α results for the optimized geometries and the barrier height of ethane is found to be comparable to that of large-basis Hartree-Fock calculations.
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  • 21
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 755-766 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It has been shown by ab initio configuration-interaction methods that the lowest 2Σ+ states of NS and SiF are ‘semidiffuse’ states, like the B 2Σ+ state of PO. The lowest 2Σ+ state of CCl also appears to be semidiffuse, although here the situation is not so clear. Semidiffuse states require diffuse orbitals in the wavefunction, but they are not Rydberg states. The second 2Σ+ state of each molecule is shown to be the lowest ns Rydberg state, whereas the third 2Σ+ state is a valence state for NS and CCl, and a 4po Rydberg state for SiF. The lowest 2δ state of each molecule derives from the 7σ → 3π valence configuration. Comparison with available experimental information shows, in general, good agreement.
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  • 22
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 779-792 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method is outlined for the calculation of the multiplet ligand-field states of transition metal complexes. The procedure involves the use of MS-Xα wave functions, in connection with irreducible tensor operators, and allows the calculation of the elements of the many-electron CI matrices. Comparison of the calculated and experimental multiplet state energies of CrF3-6, CrCl3-6, and MnF4-6 allows one to conclude that the method is useful for the prediction of ligand-field spectra of transition-metal complexes.
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  • 23
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 793-797 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report local-density functional results for the electronic structure of neutral soliton defects in polyacetylene. The results were obtained using a modified version of the discrete variational method Xα molecular cluster model. For defect-free chains the calculated X-ray and ultraviolet photoelectron spectra are in good agreement with experiment. For chains with low concentrations of solitons, we find the forbidden gap broadens slightly, and two nearly dispersionless bands, split by several tenths of eV, are introduced into the gap. The predicted splitting is in accord with recent experimental results for this defect.
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  • 24
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 817-827 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of quantum mechanical calculations are presented that suggest a number of mechanisms whereby protons may be shifted from one group to another along an H bond. The first factor to be considered is a stretching of the bond that drastically raises the energy barrier to transfer. It is possible to predict barriers for an arbitrary system based only on results for a simple system and knowledge of the relevant bond length in the isolated subsystems. Factors that increase the intrinsic basicity of the B group in A-H-B lead not only to a lowering of the energy of the A-HB state relative to AH-B but also to a reduction in the barrier to transfer of the proton from A to B. Ions in the vicinity of the H bond exert a powerful influence and can shift the proton to the less basic group across a gradient of several pK units. Rather than shielding the proton from the external ion, the H bond acts instead to amplify the effects of the electric field. Reorientation of the A and B groups relative to one another, i.e., bends of the H bond, also produce surprisingly large changes in the relative energies of the AH-B and A-HB states. Such bends are capable of pushing the proton across to the normally less basic group, providing a mechanism of coupling conformational changes to proton ‘pumping’ activity. It is found that the high and low pH states of a given H bond can have dramatically differnt relative populations of the AH-B and A-HB configurations. These observations are explained in terms of fundamental concepts involving electrostatic interaction energies.
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  • 25
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    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 29 (1986), S. 799-815 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The vibrational spectrum of uracil trapped in an argon matrix has been interpreted based on ab initio Hartree-Fock SCF calculations with a split-valence 4-21 basis set. The directly computed theoretical general valence force field was scaled with empirical scale factors in order to correct for the systematic errors originating in the limitation of the theoretical model. Scale factors transferred from related molecules provided a priori prediction of fundamental frequencies and intensities, permitting several corrections to be proposed for earlier assignments. Using the observed spectrum with the few altered assignments, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and the theoretical data. For unknown reasons, the out-of-plane force field predicted a spectrum agreeing slightly less well with experiment than did the in-plane force field. However, the overall agreement between theory and experiment provided additional support for the assumptions involved in the method. The computed force fields were compared with others available from previous work. The comparison demonstrated the importance of expanding the energy surface around the true energy minimum and of using a proper scaling procedure. Previous scaled CNDO/2 calculations were found to be surprisingly good despite the large corrections required and the fact that they were made at an incorrect geometry.
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  • 26
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    International Journal of Quantum Chemistry 29 (1986), S. 843-854 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The total-energy pseudopotential method is described, and some applications are presented with emphasis on the predictive nature of the approach. For several applications, silicon is used as a prototype material, and the successful predictions related to structure and superconductivity at high pressure are discussed.
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  • 27
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    International Journal of Quantum Chemistry 29 (1986), S. 875-881 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The experimental transition energy to the valence Ag state of butadiene is used to validate a subset of the theoretical calculations that have been carried out on this molecule. The validated calculated Ag transition energies of these calculations are then compared with the experimental Bu transition energies to determine the relative state ordering in isolated, unsubstituted butadiene, hexatriene, and octatetraene. The Ag origins are concluded to lie below the Bu origins for all polyenes except butadiene. Experimental confirmation of this conclusion is presented. The extreme breadth of the Ag transition, 1.0 eV, is noted. Some possible implications of this breadth are discussed.
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  • 28
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    International Journal of Quantum Chemistry 29 (1986), S. 883-896 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An important metabolic pathway for halogenated methanes is the detoxification reaction with glutathione in the cell cytosol fraction. Experimental studies have shown that the rate of this SN2 displacement reaction is directly related to the leaving group ability of the dissociating halide; the rate increases in order of the ions F-, Cl-, Br-, and I- leaving. In this study, we examine the role of the other halomethane substituents on the rate of this reaction for compounds with a common leaving anion. To this end, reaction Cl- + CY3Cl → ClCY3 + Cl- (Y = H,F,Cl) is examined using ab initio methods. The barrier for this exchange process may arise from requiring an inversion of the CY3 group in the transition structure, the energy required to homolytically cleave the C—Cl bond, or from unfavorable steric and electronic interactions in the five-coordinated transition structure. Of these three factors, only the third explains the calculated ordering of barrier heights. This suggests that successive halogenation of methane not only increases its activity as a substrate for anaerobic reduction, as was shown earlier, but also decreases its ability to take part in the detoxification reaction with glutathione.
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  • 29
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    International Journal of Quantum Chemistry 29 (1986), S. 897-908 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1/N expansion is used to generate semiclassical effective potentials for two-electron ions. These potentials can be considered an improvement over the conventional ones computed by Lin and Fano for the full two-electron coulomb potential insofar as they incorporate some of the quantum mechanical effects. No falling into the nucleus is allowed. Furthermore, the minima of these effective potentials are in surprising agreement with those computed by Fano, Macek, and Lin. This would indicate that the collective variable R, associated with N = 6, leads to a much smaller expansion parameter N, making the adiabatic separation more justified. The role and significance of these broken symmetry solutions with geometric localization are discussed as well as generalizations. The procedure is semiclassical in the sense that the kinetic energy becomes negligible by having a large effective mass.
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  • 30
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    International Journal of Quantum Chemistry 29 (1986), S. 909-914 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular difference densities (DD) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD. We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x-ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.
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  • 31
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    International Journal of Quantum Chemistry 29 (1986), S. 915-935 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Theoretical core effective potential methods are widely used in valence-only electron molecular calculations. These methods, which imply the frozen-core approximation, work well for the elements of the righthand side of the periodic table but are often unrealistic for metallic elements with highly polarizable cores. For these atoms one has to consider the polarization of the cores under the influence of the electric field created by the valence electrons. Moreover, relativistic corrections must be added for heavy atoms.Various theoretical approaches of core-valence interactions (polarization and core-valence correlations) will be reviewed, with a special emphasis on practical methods of calculation. The problem of handling the relativistic effects will mainly be discussed within the two-component Pauli formalism. It will be shown that the Foldy-Wouthuysen transformation is not the unique way for deriving relativistic corrections and that the second-order Dirac equation also provides a good starting point for obtaining relativistic corrections. Analytical exact results are given for the hydrogen atom. The accuracy of this approach is tested on many-electron atoms and molecules.It is finally shown that the problem of the core-valence separation is relevant to the general methodology of effective Hamiltonians that seems to provide the best promising way for filling the gap between the semiempirical and purely theoretical ab initio methods.
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  • 32
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    International Journal of Quantum Chemistry 29 (1986), S. 937-948 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The concept of an atom in a molecule in the context of density functional theory is used to analyze different levels of approximation to the description of interatomic interactions. Such an approach strongly suggests the use of Kohn-Sham atomic densities as an alternative to Hartree-Fock atomic densities in the electron gas model of Gordon and Kim. The results for rare gases and ionic crystals show that both densities lead to similar results.
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  • 33
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    International Journal of Quantum Chemistry 29 (1986), S. 949-958 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have first studied empirical regularities in various series of heteronuclear diatomic molecules between the energy E, the total number of electrons N, the equilibrium distance Re and Z̄ = (Z1Z2)1/2 where Z1e and Z2e are the nuclear charges in the diatom. In particular, for various alkali halide series, Re2|E|/N5/3 is shown to correlate rather simply with Z̄Re3. Some theoretical basis is afforded by generalizing the 1/Z expansion used early by the writers in work on homonuclear diatomics. Finally, when Z2/Z1 → ∞, a model is presented which predicts a finite asymptotic bond length and this prediction is confronted with available experimental data for both heteronuclear diatoms and for the polyatomic series CH4 to SnH4.
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  • 34
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    International Journal of Quantum Chemistry 29 (1986), S. 959-973 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The bond critical points of the binary hydrides formed by the elements of the first two rows of the periodic table have been calculated. Particular attention has been paid to the basis-set dependence of the bond critical points at the experimental equilibrium geometries, or where necessary at model geometries. With the exception of H2S, stepwise extension of the basis set leads to a smooth convergence of the bond critical points to a set of values which appear to converge to the Hartree-Fock limit. For H2S it is shownb that the position of the bodn critical point is not only more sensitive to the presence of polarization functions in the basis set, but depends strongly on the orbital exponents of the polarization functions. Extensive optimizations of the exponents of the polarization functions have been carried out with the (12s9p/5s) basis set for second-row hydrides. The effects of contracting the Huzinaga basis sets have been examined.
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  • 35
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    International Journal of Quantum Chemistry 29 (1986), S. 975-991 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular orbitals for Si2 and Ge2 have been optimized in hyper-HF calculations and utilized in valenxe CI treatments to describe the low-lying states of the molecules. The calculational results reveal pronounced similarities between the electronic structures of Si2 and Ge2. Thus, for both molecules the two lowest-lying electronic states, 3Σ-g(σ2gΠu3) and 3πu(∑g1πu3), have crossing potential energy curves, and the two lowestlying states of 1∑g+ symmetry exhibit crossing of configurations. The Sequence of the low-lying electronic states can be rationalized on basis of a simple molecular-orbital picture in which the σg and the πu valence orbitals are almost degenerate. The spectroscopic constants derived from the present work compare favorably with the results of more elaborate calculations. It appears that transition energies derived in valence CI calculations between states of identical configurations are improved in large CI calculations, whereas this is not the case for transition energies between states of different configurations. The valence CI calculations based on the molecular orbitals optimized in hyper-HF calculations appear to effer reliable descriptions of the chemical bonds as well as of the electronic structures of the molecules Si2 and Ge2.
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  • 36
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    International Journal of Quantum Chemistry 29 (1986), S. 1025-1032 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: LCGTO-VWN-MP calculations have been performed for PdH, Pd3H, and for a series of Pd10H clusters representing hydrogen chemisorbed on a (111) surface and at various sites along a diffusion path through the cluster. The calculated chemisorption geometry (over a threefold site) has a Pd—H distance of 1.72 Å, slightly longer than that proposed by Eberhardt et al. (1.69 Å) on the basis of comparison of photoemission data with slab calculations. Agreement with known energy differences is very good: We find the chemisorption site 0.25 eV (exp. 0.26) more stable than the most stable (octahedral) bulk site. The top of the diffusion barrier, a triangular site, is calculated 0.34 eV (exp. 0.23) above the octahedral site. All the sites have a split-off state of Pd d, H s character below the d bands, indicating that the ‘invisible’ state of H/Pd(111) recently (not) observed in UPS cannot straightforwardly be attributed to hydrogen incorporation at sites such as these.
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  • 37
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    International Journal of Quantum Chemistry 29 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 38
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    International Journal of Quantum Chemistry 29 (1986), S. 1043-1045 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two paradoxes are shown to exist: one in conection with spin and one in connection with computation on large (biological) molecules. Some possible mathematical and physical methods are listed and discussed that may help in the reduction of the problems.
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  • 39
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    International Journal of Quantum Chemistry 29 (1986), S. 1033-1042 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The interaction of low-energy electrons with H+2 and ligh with H2 is analyzed using the R-matrix method including both coupled state and polarization effects ab initio. Particular attention is paid to the energy region containing low-lying H*2 resonances which converge to H+2 A 2Σ+u. Resonances in both elastic scattering and the photoionization asymmetry parameter, β, are presented. At low photon eneries, satisfactory results for vibrationally resolved photoionization are obtained using the adiabatic nuclei approximation. The use of nonadiabatic techniques for higher energies is discussed.
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  • 40
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    International Journal of Quantum Chemistry 29 (1986), S. 1047-1058 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The recently developed Gaussian basis functions [2] were used in calculations on the ground electronic states of molecules containing transition-metal atoms: ScF3, TiCl4, ZrCl4, Cr(CO)6, Ni(CO)4, CuF, CuCl, Zn(CH3)2, and Cd(CH3)2. The usefulness of minimal basis sets, the importance of splitting of the valence part of the minimal basis sets, the role of the triple splitting of the d-block functions, and the need for p-, d-, and f-type polarization functions were discussed in the context of the geometrical structure and the firstorder electronic properties of the transition-metal atom compounds.
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  • 41
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    International Journal of Quantum Chemistry 29 (1986), S. 1059-1066 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results are presented of a Hartree-Fock cluster study of interstitial Ti, V, Cr, and Mn impurities in silicon. A Si10 cluster models the nearest Si atoms around a tetrahedral interstitial site in crystalline Si. The dangling bonds of the Si atoms are saturated by hydrogens. The effect of the Si core electrons is represented by an effective potential. Characteristic for the electronic structure of the low-lying states of the neutral, singly positive, and doubly positive ions in silicon is the presence of fairly delocalized but still predominantly transition-metal (3d)-like orbitals of t2 and e symmetry. For all ions the energy of the weighted average of the terms belonging to a configuration is lowest for the configuration with maximum occupation of the t2 orbitals. Ground states with maximum spin multiplicity are found for all ions, except Ti0.
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  • 42
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    International Journal of Quantum Chemistry 29 (1986), S. 1067-1076 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Use of the London-Eyring-Polanyi + 3-Center + power-series (LEP-3C-PS) analytical potential as a fit to potential energy surfaces (PES) which are known numerically only are suggested. This analytical fit was performed for the diatomics-in-molecules + 3 Center (DIM-3C) PES of HCl2 and HI2 systems. The HCl2 analytical LEP-3C-PS potential was used for classical trajectory calculations of the Cl' + HCl → HCl' + Cl reaction. The rate constant obtained from these calculations for T = 358° K is 1.95 X 109 cm3/mol sec which is close to the experimental value of 2.5 109 cm3/mol sec.
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  • 43
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    International Journal of Quantum Chemistry 29 (1986), S. 1077-1089 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Results of self-consistent field, local spin density, scattered wave calculations are reported for nickel clusters of 10, 13, and 14 atoms and these clusters interacting with one or two chemisorbed hydrogen atoms. The pure nickel clusters all have a reasonable average atomic magnetic moment (the average over all the clusters is 0.66µB) and the addition of hydrogen reduces this moment in each case. The reduction of magnetic moment is clearly larger on the nickel atoms that are nearest to hydrogen but there is also a noticeable change in the moments of the other atoms of the clusters. Three factors, of varying importance for the different clusters, contribute to the changes in the overall and local magnetic moments: (i) The extra electron brought in with the hydrogen goes into a down-spin Ni d level, reducing the moment. (ii) The reelectron duced moment is accompained by a reduced exchange splitting and consequently some up-spin d electrons, not directly involved in the bonding to hydrogen, are transferred to lower lying down-spin d orbitals. (iii) For atoms close to the adsorbate, d character in the local density of states is pushed above the Fermi level through antibonding interactions with the hydrogen, further reducing the moments of these atoms.
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  • 44
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    International Journal of Quantum Chemistry 29 (1986), S. 1105-1116 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Potential energy surfaces (PES) for Na(2S, 2P) interacting with a tungsten surface partially covered with sodium ions are computed within the framework of the diatomics-in-molecules (DIM) method. A small number (1 to 10) of adsorbed sodium atoms are considered explicitly, the effect of the rest being taken into account through the fragment matrix elements in the DIM formulation. A physical model proposed previously to account for the experimental observation of Na+ and Na+2 ions is supported by these calculations and, in addition, a new pathway to Na+2 products is identified. The effect of including extra adsorbed atoms is discussed in terms of the molecular wave functions and a sensitivity analysis.
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  • 45
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    International Journal of Quantum Chemistry 29 (1986), S. 1091-1104 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: LCGTO-LSD model potential calculations have been performed for CO interacting with two-, four-, and eight-atom clusters of Pd, chosen to model the bridge site of the (100) surface. The geometry and vibrational frequencies are not very sensitive to the cluster size. For Pd8 + CO we obtain dC - O = 1.18 Å (1.13 ± 0.1 exp.), dPd - C = 1.87 Å (1.93 ± 0.07 exp.), and (uncoupled) estimates for ωC - O = 1828 cm-1 (1895 exp.) and ωPd - CO = 454 cm-1 (339 exp.) Binding energies of 4.8, 3.8, and 2.6 eV are calculated, respectively, for Pd2 + CO, Pd4 + CO, and Pd8 + CO which may be compared with the experimental initial heat of adsorption of 1.6 eV. Ionization potentials for CO-derived levels are in excellent agreement with experiment (relative to ∊F: 4σ (-11.0 eV, -11.2 exp.); 5σ (-8.0, -8.2 exp.); 1π [-7.5 (b1), -7.3 (b2), -7.5 exp.]). The main negative ion states of 2π* character are calculated at 2.8 eV (b1) and 2.7 eV (b2) above EF. Other states with appreciable 2π* character are found near 5 eV. These may be compared with inverse photoemission results which show a broad peak centered at 4.8 eV. Interactions of the 4σ, 5σ, 1π, and 2π* orbitals of CO with the metal are discussed. The 4σ and 5σ levels are highly mixed, each receiving appreciable contributions from the 4σ and 5σ orbitals of isolated CO. This is discussed in relation to the dispersion of the 4σ and 5σ levels observed in UPS and to the photon-energy dependent intensities of the 4σ and 5σ resonances. The 2π* component of the backbonding comes through several levels in the upper part of the d band which contain small 2π* contributions in bonding combination with Pd d orbitals. The main 2π* orbitals (contaminated by small antibonding contributions from the metal) are empty (see above).
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  • 46
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    International Journal of Quantum Chemistry 29 (1986), S. 1127-1137 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The IR frequencies of cyclopropene and methylenecyclopropene were computed with inclusion of correlation (MP2/6-31G*). These along with those of ethylene obtained earlier by Pople were compared with SCF results (DZ + P). It was found that the MP2 frequencies agreed better with experiment, but that the order of the bands remained the same as those computed without correlation.
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  • 47
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    International Journal of Quantum Chemistry 29 (1986), S. 1117-1126 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electron transmission spectroscopy (ETS) and bound-state and continuum multiple-scattering Xα (MS-Xα) calculations are employed to characterize the unoccupied a′2, a′1 and e′ orbitals of BF3 and BCl3. The a′2 orbital of BF3, which produces a peak about 7 eV below threshold in the x-ray absorption spectrum (XAS), generates a scattering resonance at 3.5 eV in ETS. Similarly, the e′ orbital that lies about 2 eV above threshold in XAS occurs about 13-16 eV above threshold in ETS. Dissociation of F- from BF3 due to electron attachment is attributed to a core-excited shape resonance involving an e″ → a″2 excitation and electron capture into the a′2 orbital. In BCl3 all the unoccupied orbitals lie at lower energy than in BF3 and are closely spaced, making definitive spectral assignments difficult. Both Hartree-Fock (HF) and MS-Xα methods apparently underestimate the stability of the unoccupied e′ orbital of BCl3. Vibronic coupling due to out-of-plane bending may significantly affect the spectral intensities. Feshbach resonances are observed for BCl3 at energies close to those observed in the vacum-UV absorption spectrum. The lower energies of the unoccupied orbitals of BCl3 are consistent with their stronger bonding to nucleophiles.
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  • 48
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    International Journal of Quantum Chemistry 29 (1986), S. 1139-1146 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 4 Ill.
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  • 49
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    International Journal of Quantum Chemistry 29 (1986), S. 1147-1154 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The peculiar properties of the three-membered ring in cyclopropane are partially due to characteristic relaxation effects of the carbon orbitals in the molecule. AO contraction in the two strongly C—C bonding MO's of A'1 type is essential for the stability of the C3 ring, whereas AO expansion in the E'-type HOMO contributes to the π character of the peripheral C—C bonds.
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  • 50
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    International Journal of Quantum Chemistry 29 (1986), S. 1155-1164 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In a previous paper we have shown that the long-range potential generated by the two-center distributions can be written as a sum of two multipolar expansions centered at the terminal atoms and defined so that the two series of multipoles be minimal. In this paper we show that both charge distributions and short-range potentials can also be separated in atomic contributions in a way consistent with the above mentioned partition of the long-range potentials.
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  • 51
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    International Journal of Quantum Chemistry 29 (1986), S. 1177-1180 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The preferred conformations of the thyrotropin-releasing hormone (TRH) have been calculated by the global optimization method proposed earlier by us. [G. Subba Rao, R. S. Tyagi, and R. K. Mishra, J. Theor. Biol. 90, 377 (1981)]. The potential function used comprises the electrostatic, nonbonded, torsional and hydrogen-bonding terms. The results are in good agreement with the crystal structures of TRH. No intramolecular hydrogen bonding is found to occur.
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  • 52
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    International Journal of Quantum Chemistry 29 (1986), S. 1209-1222 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Tests of the local-space approximation for electronic structure are carried out on a variety of hydrogenbonded systems. Regardless of size they are all found to be well localized in the sense of this approximation. For a local space defined by the valence atomic orbitals on X—H ··· Y, one obtains a bond energy within 6% of the “exact” full-space value. The potential energy surface and the electronic charge distribution are also accurately determined. In contrast with cluster models the local-space approximation satisfactorily accounts for long-range electrostatic and polarization effects.
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  • 53
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    International Journal of Quantum Chemistry 29 (1986), S. 1339-1349 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The elastic, inelastic, and total intensities for x-ray and high-energy electron scattering from the 10-electron systems Ne, HF, H2O, NH3, and CH4 have been calculated by using SCF-MO wave functions obtained with double-zeta quality bases of Gaussian contractd wave functions. The effect of molecular binding and various other trends and systematics in the intensities have been examined with the help of difference functions computed between the present scattering intensities and those for the indpendent atom model (IAM).
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  • 54
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    International Journal of Quantum Chemistry 29 (1986), S. 1351-1364 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED. A detailed analysis of the electronic factors involved in this process is presented.
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  • 55
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    International Journal of Quantum Chemistry 29 (1986), S. 1365-1372 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A novel approach to the impurity dipole relaxation in polar solids is described in which the dipole reorientational sites are formed by an appropriate electron - phonon coupling interaction. The electronic states used for a two-site formulation are defined as ones pertaining to the impurity and/or the associated intrinsic point defect. The theory takes into account the adiabaticity of the electron transfer in addition to the barrier-controlled ionic tunneling. It compares favorably with available experimental relaxation-time data on alkali halides.
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  • 56
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    International Journal of Quantum Chemistry 29 (1986), S. 1373-1382 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Several Fe(H2O)n2+ clusters, with n up to 20, have been studied, both by energy minimization in the pairwise approximation and by Monte Carlo simulation. In the last case the calculations have been carried out at three different temperatures in order to investigate the effect of thermal agitation. The most interesting result which can be deduced from this work is the existence of eight water molecules in the first hydration shell of the iron (II) ion. A microscopic analysis has shown that the minimum energy structure of the Fe(H2O)82+ cluster presents a D4d symmetry. This structure is slightly distorted as far as the temperature is increased. The validity of these theoretical predictions is discussed.
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  • 57
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    International Journal of Quantum Chemistry 29 (1986), S. 1383-1396 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The present work reviews a systematic spectroscopic investigation of water in the single crystal K4Fe(CN)6 · 3H2O (KFCT) and its isotopic solid solutions. On the basis of precise polarization measurements of Raman spectra and infrared studies, the positions of the OH(OD) oscillators in the ordered crystal state (T 〈 150 K) are determined. Overtones and combinations of water vibrations in the near infrared region are studied that allow for the evalution of the coupling and anharmonicity constants of the modes. The isotopic dilution technique is used in order to investigate the influence of intra- and intermolecular interactions on the OH(OD) vibrations. Information about the distortion of the water molecules, as well as many correlations between spectral parameters of OH(OD) stretchings and structural characteristics of H bonds in KFCT, are derived. As a result, a complete picture of the vibrational dynamics and the structure of the crystallization water in KFCT is obtained.
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  • 58
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    International Journal of Quantum Chemistry 29 (1986), S. 1397-1428 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The experimental technique of electron momentum spectroscopy (EMS) (i.e., binary (e, 2e) spectroscopy) is discussed together with typical examples of its applications over the past decade in the area of experimental quantum chemistry. Results interpreted within the framework of the plane wave impulse and the target Hartree - Fock approximations provide direct measurements of, spherically averaged, orbital electron momentum distributions. Results for a variety of atoms and small molecules are compared with calculations using a range of Fourier transformed SCF position space wavefunctions of varying sophistication. Measured momentum distributions (MD) provide a “direct” view of orbitals. In addition to offering a sensitive experimental diagnostic for semiempirical molecular wavefunctions, the MD's provide a chemically significant, additional experimental constraint to the usual variational optimization of wavefunctions. The measured MD's clearly reflect well known characteristics of various chemical and physical properties. It appears that EMS and momentum space chemistry offer the promise of supplementary perspectives and new vistas in quantum chemistry, as suggested by Coulson more than 40 years ago. Binding energy spectra in the inner valence region reveal, in many cases, a major breakdown of the simple MO model for ionization in accord with the predictions of many-body calculations.Results are considered for atomic targets, including H and the noble gases. The measured momentum distribution for H2 is also compared with results from Compton scattering. Results for H2 and H are combined to provide a direct experimental assessment of the bond density in H2, which is compared with calculations. The behavior of the outer valence MD''s for small row two and row three hydride molecules such as H2O and H2S, NH3, HF, and HCl are consistent with well known differences in chemical and physical behavior such as ligand-donor activity and hydrogen bonding. MD measurements for the outermost valence orbitals of HF, H2O and NH3 show significant differences from those calculated using even very high-quality wavefunctions. Measurements of MD's for outer σg orbitals of small polyatomic molecules such as CO2, COS, CS2, and CF4 show clear evidence of mixed s and p character. It is apparent that EMS is a sensitive probe of details of electronic structure and electron motion in atoms and molecules.
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  • 59
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    International Journal of Quantum Chemistry 29 (1986), S. 1437-1455 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Different methods to take into account many body effects in photoionization processes are discussed. Numerical results are shown for some indirect methods, several approximations within the Green's function formalism, and for potential models. The suitability of the Green's function method for the investigation of satellite structure is considered.
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  • 60
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    International Journal of Quantum Chemistry 29 (1986), S. 1457-1462 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The appearance of the maximum linewidth by the vibrational predissociation of I2He van der Waals (vdW) molecule as a function of the initial vibrational quantum number of I2 is predicted by the dynamic perturbation theory. The kinetic energy perturbation is introduced in addition to perturbation potential. The linewidth agrees quite well with experiment.
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    International Journal of Quantum Chemistry 29 (1986), S. 1463-1482 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a new parametrization (based on ab initio calculations) of the bending potentials for the two lowest potential energy surfaces of the reaction O(3P) + H2, and we use it for rate constant calculations by variational transition-state theory with multidimensional semiclassical tunneling corrections. We present results for the temperature range 250-2400 K for both the rate constants and the intermolecular kinetic isotope effects for the reactions of O(3P) with D2 and HD. In general, the calculated rate constants for the thermal reactions are in excellent agreement with available experiments. We also calculate the enhancement effect for exciting H2 to the first excited vibrational state. The calculations also provide information on which aspects of the potential energy surfaces are important for determining the predicted rate constants.
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  • 62
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    International Journal of Quantum Chemistry 29 (1986), S. 1483-1492 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Standard Lippmann-Schwinger theory does not apply to decay of metastable or unstable states due to failure of the adiabatic hypothesis. In the Fock-Tani representation, discrete unstable states can be described by state vectors orthogonal to the asymptotic states representing their decay fragments, and a decay/formation interaction is exhibited explicitly as a portion of the total interaction Hamiltonian. This allows a straightforward derivation of a generalized Lippmann-Schwinger “half-collision” differential decay cross section without the need of projection operators. It reduces in first order to the product of a resonance line shape by a Golden Rule matrix element squared. In the general case the line shape is non-Lorentzian and the matrix element factor contains final-state interaction contributions. The exact expression is expected to be applicable to a variety of processes such as predissociation, autoionization, and Auger effect. The derivation employs the van Hove resolvent formalism to exhibit the dependence of the cross section on the complex self energy of the decaying state.
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  • 63
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    International Journal of Quantum Chemistry 29 (1986), S. 1493-1502 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In muon spin rotation (μSR) experiments, where spin-polarized positive muons are stopped in condensed matter, three magnetically distinguishable chemical environments can be observed. That is, the Larmor frequencies associated with diamagnetic environments and two types of paramagnetic environments (muonium and radicals) can be resolved. The chemical identities of the latter two are distinct since their Larmor frequencies are distinct, whereas the chemical identities of the possible diamagnetic species are not determined by the Larmor frequency since chemical shifts can not be resolved in μSR experiments. However, two different diamagnetic species have been observed in experiments performed on mixtures of noble gases. Their distinction arises through different thermal rate constants that lead to “fast” and “slow” relaxing components of the diamagnetic signal. The pressure dependencies of the amplitudes associated with these components are related to the stopping dynamics of muons in noble gas targets. A set of coupled rate equations for muon spin dynamics, based upon quantal Boltzmann equations, have been developed to describe this process in single component gases. This theory is now extended to mixtures. In particular, the dynamics of the muon spin is generated by the muonium hyperfine interaction and by time dependent rate constants for the various chemical species that are assumed to be present, namely, muonium and three diamagnetic species. Radicals have not yet been observed in low pressure gases. The coupled quantal rate equations are solved for two models of the stopping dynamics wherein the rates are taken as square box functions of time.
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  • 64
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    International Journal of Quantum Chemistry 29 (1986), S. 1503-1512 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of 4-H-pyran-4-one and its sulfur analogues were studied using ab initio wave-functions. Bond lengths and overlap populations suggest low aromaticity for this group of compounds. Examination of Jorgensen plots of the lowest π orbitals of I--IV leads to the aromaticity order 4H-thiopyran-4-thione (IV) 〉 4H-thiopyran-4-one (II) 〉 4H-pyran-4-thione (III) 〉 4H-pyran-4-one (I). The effects of including d orbitals were studied using the 3-21G, 3-21G* (6d), and 3-21G* (5d) basis sets. Optimized bond lengths, vibrational frequencies, ionization energies, and dipole moments were also obtained, and results for different basis sets were compared.
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  • 65
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    International Journal of Quantum Chemistry 29 (1986), S. 1525-1533 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We investigate the question of the existence or nonexistence of mobility edges that separate the localized and extended states in two-dimensional disordered electron systems treated within the Anderson model. Evidence is produced to show that the mobility edges exist if the amount of disorder is less than a critical value. The following three different agruments are presented: (1) exact enumeration of self-avoiding walks that contribute to the renormalized perturbation series of self-energy whose convergence (divergence) indicates localization (delocalization); (2) “quantum percolation” in a strongly disordered binary alloy; and (3) influence of magnetic field on localized and extended states (the quantum Hall Effect problem).
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  • 66
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    International Journal of Quantum Chemistry 29 (1986), S. 1705-1712 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The omega model is reviewed in the case of a three-electron function and the omega function determined for the lithium atom ground state by using a procedure based on Brillouin' theorem. The resulting function is analyzed in terms of natural configurations, and the correlation coefficient determined as a function of the distance to the nucleus. It is found that the omega function is essentially equivalent to two configurations built up with the occupied NO' and that the model introduces mainly correlation at large distance. These results contrast with those obtained for the beryllium atom using the same model.
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  • 67
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    International Journal of Quantum Chemistry 29 (1986), S. 1687-1695 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that the exclusion principle merely requires the electrons to be divided into two distinct groups, then, according to our assumption of the distinguishability of electrons, its mathematical representation can be the spatial double antisymmetrization or the spatial-spin antisymmetrization; in the latter case an auxiliary spin double symmetrization is also required in order to distinguish the two types of electrons. The second requirement is used to recognize the fact that, although all electrons can be assumed to be indistinguishable under certain condition, their natural characteristic of two distinct groups must still be preserved.It is further shown that the double antisymmetrization (or double symmetrization) determines the spin quantum number M, whereas the Löwdin spin projector for nonorthogonal eigenfunctions and the Wigner matric basis for orthogonal eigenfunctions will decide the spin quantum number S, Both the spatial double antisymmetrizer and the spin double symmetrizer will reject all the unnecessary eigenfunctions and project out only one unique set of linearly independent eigenfunctions.
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  • 68
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    International Journal of Quantum Chemistry 29 (1986), S. 1697-1704 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Because it is useful to have the molecular electrostatic potential as an element in a complex scheme to assess the toxicity of large molecules, efficient and reliable methods are needed for the calculation and characterization of these potentials. A multicenter multipole expansion of the molecular electron charge density calculated with a limited Gaussian basis set is shown here to have only a finite number of nonzero terms from which the molecular electrostatic potential can be calculated. The discrete contributions to the electrostatic potentials from the terms of this expansion provide a physically meaningful decomposition of the potential and a means for its characterization. With pyrrole as an example, the electrostatic potential calculated from this finite expansion of the electron density is compared to that obtained from exact calculations from the same wave function. Good agreement is obtained at distances greater than 1.5 A from any atom in the molecule. In contrast, rearrangement of the terms into an expansion corresponding only to Mulliken atomic charges and dipoles yields a decomposition that produces electrostatic potentials which agree less well with the exact potential. This discrepancy is attributable to the neglect of terms due to higher moments.
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    International Journal of Quantum Chemistry 29 (1986), S. 1713-1734 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the total atomic Thomas-Fermi (TF) energy many expressions in terms of the kinetic and potential energy contributions can be given. Thirty of these expressions exhibit either a maximum or a minimum if some variational approximation to the TF function is used. Three of these expressions, to note E, G, and J (see text) have been used in an optimization procedure, in which four two-parameter and two three-parameter approximate TF functions have been considered. One-parameter functions cannot be optimized, as the one parameter must be fixed to ensure proper normalization. It is found that optimization of E and G give reasonable and similar results, whereas the results of optimization of J are generally not very impressive. Where possible, expectation values of the type 〈rn〉 (with n = -1, 1, 2, and 3) have been calculated from ten approximate TF functions. A new estimate of the exact atomic TF energy, as well as of the derivative of the TF function at the origin, has been obtained.
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    International Journal of Quantum Chemistry 29 (1986), S. 1735-1743 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Formulas are derived which allow the direct construction of total orbital angular momentum eigenfunctions for many-particle systems without the use of Clebsch-Gordan coefficients. One of the equations is closely analogous to Dirac' identity for the total spin operator. This equation describes the action of L2 on a function of the particle coordinates in terms of a class operator of the symmetric group and a "contraction operator." A general projection operator for constructing symmetric eigenfunctions of L2 is presented.
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  • 71
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    International Journal of Quantum Chemistry 29 (1986), S. 1745-1754 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural features of a series of tetraatomic carbonyls in the 1,3nπ* states are studied by using a recently proposed MCSCF algorithm at the INDO/2 level. The computed geometrical parameters and the inversion barrier heights agree quite well with the available experimental or ab initio theoretical data. The calculated transition energies turn out to be somewhat lower than the experimental counterparts. The specific MCSCF algorithm used is found to have excellent convergence properties.
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  • 72
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    International Journal of Quantum Chemistry 29 (1986), S. 1755-1774 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The symmetry properties of the mono- and bielectronic terms contributing to the Fock matrix in the ab initio Hartree-Fock treatment of periodic systems are discussed. A computational scheme which takes full advantage of the point symmetry is presented; in this respect, it represents a generalization of the scheme proposed in Int. J. Quantum Chem. 17, 501 (1980). Computational details and numerical examples are reported; it is shown that with respect to two of the bottlenecks of this kind of calculation, namely, computer time and backing storage required for the bielectronic integrals, it is possible to obtain saving factors as large as h and h2, respectively, where h is the order of the point group. Preliminary tests are reported which indicate that the study of relatively complicated systems, like quartz or corundum (9 and 10 atoms in the unit cell, respectively) at an ab initio Hartree-Fock level is now within reach.
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  • 73
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    International Journal of Quantum Chemistry 30 (1986), S. 97-107 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometrical and electronic structures and the 1H-chemical shifts of some conjugation deficient hydrocarbons and their benzenoid isomers 1-3 which consist of two concentric perimeters have been studied by means of semiempirical procedures. Conclusions on the importance of macrocyclic annulenoid conjugation are drawn from the bond lengths pattern and the proton shifts in these compounds.
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    International Journal of Quantum Chemistry 30 (1986), S. 127-136 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio MO computations have been carried out for the crystalline form of the B6 vitamin, pyridoxinium chloride. In the crystal, the pyridoxine is protonized to pyridoxinium, with the formation of a hydrogen bond N—H+ ⃛ Cl-. For an isolated molecule, the calculations predict a single potential well, with H+ placed close to Cl- at the minimum. When neighboring molecules are included in the calculations, a double well is formed, and the lower minimum occurs for the proton placed near the experimentally observed position, in the vicinity of the N atom.Mulliken populations, which depend on the proton position, are described.
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  • 75
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    International Journal of Quantum Chemistry 30 (1986), S. 239-264 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The necessary theory to construct different direct orbital optimization schemes in the framework of the spin-projected Hartree-Fock (spin-PHF) method for systems with an even number of electrons and for the proper characterization of the pertinent spin-PHF solutions is developed using the formalism of second quantization. The treatment is based on a unitary transformation of a reference DODS (different orbitals for different spins) Slater determinant. The energy expectation value corresponding to the transformed DODS Slater determinant is expanded in a Taylor series about the reference DODS Slater determinant, including second-order terms. On the basis of this expansion we propose several direct iterative approaches to optimize the spin-PHF energy expectation value: for example, implementations of steepest descent and conjugated gradient techniques, and Newton-Raphson, Fletcher, and limited configuration interaction approaches. The analysis of the second-order term in the Taylor series expansion of the spin-PHF energy expectation value about a spin-PHF solution is used to define stability conditions for spin-PHF solutions. Explicit expressions are reported for all matrix elements between different spin-projected DODS Slater determinants appearing in the treatment.
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  • 76
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    International Journal of Quantum Chemistry 30 (1986), S. 147-151 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Simple sum-rules that arise in connection with infinitesimal transformations have been set up and studied with respect to their use in assessing the quality of resonance calculations for cases in which no independent information on the metastable state is available.
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  • 77
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A graph representation of terms in the gradient expansion of the kinetic energy density functional is presented. We briefly discuss the implications of the virial theorem for the graph structure and relations between possible graphs at a given order of expansion.
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  • 78
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Additional Material: 1 Ill.
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    International Journal of Quantum Chemistry 30 (1986), S. 347-351 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ground state one-electron energies have been calculated for the substitutional and interstitial helium, neon, and argon impurities in silicon by use of the self-consistent-field multiple-scattering Xα method within the framework of the Watson-sphere-terminated molecular cluster model. The substitutional noble gas atoms are found to inhibit the rebonding of the first neighbor's dangling bonds through orthogonality repulsion. The interstitial nobel gas atoms induce empty shallow levels close to the bottom of the conduction band. We tentatively relate these levels to the emission bands observed for silicon crystals implanted with helium, neon, and argon.
    Additional Material: 2 Ill.
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  • 80
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    International Journal of Quantum Chemistry 30 (1986), S. 353-366 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Self-consistent local density theory is used to calculate the electronic structure associated with impurities and defects in transition metals and their oxides. An embedded molecular cluster scheme is used in which 15-30 atoms are treated explicitly by the LCAO discrete variational scheme, and the surrounding environment is represented by a potential field. One-electron spectroscopic properties are discussed in terms of densities of states and population analyses; emphasis is placed upon detectable features induced by the presence of defects or impurities. Hyperfine fields and local magnetic moments are examined for binary transition metal alloys, and the effects of local clustering are simulated for comparison with Mössbauer, NMR, and neutron magnetic scattering data. The influence of interstitials and vacancies on metal X-ray absorption near edge spectra (XANES) of the monoxides is evaluated and used to interpret features of the M1-xO K edge spectra. The energy of formation of isolated cation vacancies in Fe1-xO is calculated, and the binding energies of several plausible (m:n) vacancy-interstitial metal defect clusters are presented. We give some semiquantitative explanations for the relative stability of different defect configurations.
    Additional Material: 6 Ill.
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  • 81
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    International Journal of Quantum Chemistry 30 (1986), S. 367-376 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Relations between wave-mechanically calculated moments of momentum 〈pn〉, with n = 2, 1, and - 1, for some light diatoms are first compared with electron density approximations given by In = ∫{ρ(r)}1 + n/3. Whereas the ratios 〈pn〉/In are predicted by semiclassical theory as 5.74, 2.32, and 0.48, respectively, the calculated values with densities of Hartree-Fock-Roothaan quality are 6.36, 2.38, and 0.42, respectively. Polyatomics, including the almost spherical XHn molecules, are then considered, with some attention focusing on methane, followed by a comparison between some empirical results for the Compton profile of hydrocarbons and the bond profile C—H related to the methane Compton line shape. Some understanding of the empirical results of Eisenberger and Marra is thereby gained. After a brief discussion of momenta in graphite, we consider the electron-electron pair function of polyacetylene in momentum space in relation to that in a three-dimensional electron liquid, such pair correlation being related to plasmon dispersion.
    Additional Material: 3 Tab.
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  • 82
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    International Journal of Quantum Chemistry 30 (1986), S. 495-506 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Variational transition state theory with semiclassical ground-state transmission coefficients is applied to calculate reaction rate constants, activation energies, and kinetic isotope effects for the protium (H) and muonium (Mu) cases of H + F2 → HF + F and H + Cl2 → HCl + Cl. We examine four potential energy surfaces for H + F2 and two for H + Cl2. For H + F2 there is an unresolved difference between the available experiments; the calculations are consistent with the Mu experiments of Fleming and coworkers and with the H experiment of Albright et al. but not with the H experiment of Homann et al. For H + Cl2 the calculations based on the surface of Polanyi and coworkers are in good agreement with experiment, but those based on the surface of Last and Baer are not. For both reactions the Mu rate constants and the kinetic isotope effects are greatly enhanced by tunneling.
    Additional Material: 4 Tab.
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  • 83
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    International Journal of Quantum Chemistry 30 (1986), S. 541-546 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The introduction of an appropriately chosen imaginary potential in the physical asymptotic region may transform a resonance wave function into an integrable function. We examine the conditions under which this procedure and the method of exterior complex scaling become equivalent. The procedure is tested in multichannel situations describing rotational predissociation of a van der Waals complex and Stark ionization of the hydrogen atom.
    Additional Material: 2 Tab.
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  • 84
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    International Journal of Quantum Chemistry 30 (1986), S. 767-768 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The formal structure of density functional theory should include a third theorem stating that: “the internal symmetry of the system, through a set of parameters {Ci.N}, is a necessary part of the theory”. The variational equations should read δE[ρ;{Ci.N}] = 0, and the parameters Ci.N included either in the construction of the density function (as in the Kohn-Sham procedure) or in the energy functional itself.
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  • 85
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    International Journal of Quantum Chemistry 30 (1986), S. 772-772 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 86
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    International Journal of Quantum Chemistry 30 (1986), S. 773-785 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Collisional time-correlation functions (TCFS) combine methods of scattering theory and of many-body theory to describe the interaction of projectiles with complex targets. In recent work, we have developed the theory of collisional TCFS for molecular interactions using multiple scattering expansions, semiclassical approximations and operator algebras. The theory has been applied to atomic collisions with molecules, to calculate cross sections and compare them with experiment.
    Additional Material: 1 Ill.
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  • 87
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    International Journal of Quantum Chemistry 29 (1986), S. 1775-1778 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Let f1, ⃛, fn be a basis of unit vectors and g1, ⃛, gn an orthonormal basis in a Hilbert space. We consider and solve the problem of finding an orthonormal basis e1, ⃛, en such that a weighted average of the distance of ej from fj and gj is minimized in the sense of least squares.
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  • 88
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    International Journal of Quantum Chemistry 29 (1986), S. 1849-1849 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 89
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    International Journal of Quantum Chemistry 30 (1986), S. 1-5 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electron densities and expectation values for several physical properties predicted by three single determinant wave functions for the beryllium atom are presented. The wave functions are the Hartree-Fock, the Best Density Matrix, and the X-ray fitted. The X-ray fitted electron density comes closest to reproducing the exact density for r 〈 8.0 a.u. and gives, in most cases, better values for the physical properties.
    Additional Material: 2 Ill.
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  • 90
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new method for studying different contributions to the magnetic shielding tensor is proposed. These contributions come from different molecular fragments, which are defined through a localization procedure. They are obtained by means of inner projections of the polarization propagator onto the subspace defined by the chosen molecular fragment. To illustrate the utility of this method, results are presented for the proton desheilding effect of steric crowding. Different degrees of approximation both in the polarization propagator formalism and in the ground state wave function can be used. Results presented in this paper were obtained at the INDO level, within the CHF scheme. Good agreement with experimental values is obtained.
    Additional Material: 4 Tab.
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  • 91
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    International Journal of Quantum Chemistry 30 (1986), S. 109-118 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Theoretical calculations for polaron-type defect structures on organic conjugated polymers are carried out by Green's function and transfer function formalism. Firstly, the renormalization concept is applied to mathematically convert the polymers PPP, PPY, and PTP to an equivalent (trans-PA)-like form. Then, it is shown that Green's functions and polaron wave functions are readily obtained from the equations for trans-PA by the appropriate substitution of the renormalized parameters. It is suggested that the existence of defects such as polarons or bipolarons could be invoked to provide a unified treatment of electric conduction in organic conjugated polymers.
    Additional Material: 5 Ill.
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  • 92
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    International Journal of Quantum Chemistry 30 (1986), S. 119-126 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new definition of the MO-LCAO effective atomic charge is proposed, based on the concept of the topo-logical atom A general equation for the MO-LCAO electron occupancy of a topological atom is derived. It is shown that the intuitively introduced Mulliken gross atomic population is in fact an approximation to the electron occupancy of the corresponding topological atom. An approximate semiempirical procedure for evaluating the effective charge and effective radius of the topological atom is discussed.
    Additional Material: 1 Ill.
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  • 93
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 94
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    International Journal of Quantum Chemistry 30 (1986), S. 143-153 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The n-level quantum system density-operators evolution is not usually given by the Liouville equation, but by a more general one. This equation must keep density-operators trace, hermiticity, and positiveness. These three conditions restrict the available kinds of evolution equations. In this paper we investigate linear equations for systems without memory effects. By using the first two conditions and the formalism we introduced in earlier papers, the evolution equation takes the form of a first order differential equation concerning the n2 - 1 dimension “coherence-vector.” The third one is the essential object of this paper. Moreover, we obtain a cononical splitting of this equation into four parts that may be separately studied.
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  • 95
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    International Journal of Quantum Chemistry 30 (1986), S. 413-420 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio SCF and CI calculations using a double-zeta plus polarization basis set have been carried out on the trichlorine radical Cl3 to determine its electronic structure. The minimum in energy is determined for a bent structure at a bond angle of 146° and bond lengths of 2.18 Å (SCF) or 2.22 Å (CI). At linear geometry a 2Πu state is found to be lowest, approximately 7 kcal above the bent minimum, followed by a 2∑g+ state, which is around 4 kcal higher. This situation suggests that already for low quantum numbers a complex vibrational pattern in the Cl3 infrared spectrum is to be expected due to spin-orbit coupling as well as coupling of electronic, vibrational, and rotational motion.
    Additional Material: 2 Ill.
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  • 96
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    International Journal of Quantum Chemistry 30 (1986), S. 433-436 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this letter we show that the calculation of analytic second derivatives of variational potential energy surfaces with respect to nuclear coordinates is a minimization problem.
    Additional Material: 1 Tab.
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  • 97
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    International Journal of Quantum Chemistry 30 (1986), S. 449-449 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 98
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    International Journal of Quantum Chemistry 30 (1986), S. 451-451 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 99
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    International Journal of Quantum Chemistry 30 (1986) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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  • 100
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    International Journal of Quantum Chemistry 30 (1986), S. 467-477 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ketonization of vinyl alcohol in aqueous solution has been studied by means of ab initio calculations, using the STO-3G basis set. The intervention of a chain of two water molecules reduces dramatically the potential barrier. This result agrees with experimental facts.
    Additional Material: 4 Ill.
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