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  • 11
    Electronic Resource
    Electronic Resource
    A method for selection and use of numerical integration points for molecules with cylindrical symmetry (1973)
    Springer
    Theoretical chemistry accounts 31 (1973), S. 201-204 
    Details
    ISSN: 1432-2234
    Keywords: Transcorrelated method ; Numerical integration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Boys and Handy [1] have discussed the solution of the bivariational equations with restricted numerical integration. One of the weaknesses of the method was that in the numerical summations over points, some points arose with r ij= 0 and non-zero weights. This makes the method quite impractical for the Schrodinger Hamiltonian (because of the singularity at r ij= 0), and it cannot be advantageous for the transcorrelated Hamiltonian C−1HC because there will be some discontinuous higher derivatives at r ij=0. Here it is shown how the symmetry of cylindrically symmetric molecules can be used to eliminate such points, without losing any of the advantages of the overall method, such as the convergence of the eigensolutions. It is also shown how the primary numerical integration points (z i, ri) may be chosen in any calculation such that each is associated with an equal amount of one-electron density. The choice of the angular coordinates are governed by the removal of the r ij=0 points and maintaining the natural orthogonality between orbitals of different symmetry types. The method has been programmed and found to be practical, although no new molecular calculations have yet been performed. It is to be hoped that these points will give a basis for new transcorrelated calculations on diatomic molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526509
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  • 12
    Electronic Resource
    Electronic Resource
    Integration points for the reduction of boundary conditions (1973)
    Springer
    Theoretical chemistry accounts 31 (1973), S. 195-200 
    Details
    ISSN: 1432-2234
    Keywords: Numerical integration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An analysis of a method for the numerical evaluation of the integral $$\int\limits_a^b { f\left( x \right)} dx$$ is presented. The method introduces a change of variable, x = x(q), with the property that d nx/dqnis zero at x = a, x = b for n = 0, 1, 2,... N, where N is an integer to be chosen. The Euler-Maclaurin formula shows that the resulting integral in the variable q is ideally suited for numerical integration, using equally spaced points and equal weights in q-space. Examples are given for various integrals which occur in quantum chemistry and applications to more than one dimension are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526508
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  • 13
    Electronic Resource
    Electronic Resource
    On the convergence of the Møller-Plesset perturbation series (1985)
    Springer
    Theoretical chemistry accounts 68 (1985), S. 87-100 
    Details
    ISSN: 1432-2234
    Keywords: Perturbation series convergence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Møller-Plesset perturbation energies, up to as much as 48th order, have been calculated for H2O (RHF and UHF framework) and NH2 (UHF framework) atC 2v geometries (r e , 1.5r e and 2r e ). Atr e , the RHF and UHF series rapidly converge, but at2r e , the RHF series converges erratically with an energy at 43rd order within 10−7 hartree of the exact value whereas the UHF series converges smoothly, but very slowly, and for H2O has an error in excess of 10−5 hartree at 48th order. The significance of these results is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00698753
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  • 14
    Electronic Resource
    Electronic Resource
    Disilene, silylsilylene and their cations (1986)
    Springer
    Theoretical chemistry accounts 70 (1986), S. 393-406 
    Details
    ISSN: 1432-2234
    Keywords: Disilene ; Silylsilylene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract An accurate examination of features of the ground state surfaces of Si2H4 and Si2H 4 + is reported; they are compared to C2H4 and C2H 4 + . For the neutral species, accurate SCF calculations show disilene to be planar, but silylsilylene has the lower energy, whereas at the correlated (CI, MP2, MP3, MP4(SD)) levels disilene becomes trans bent and has the lower energy by ≈6 kcal/mol. In view of a recent theoretical suggestion that this value should be 23 kcal/mol, we have used large basis sets in these investigations. Our calculations cannot support this large value. Similar investigations are reported for the cation, where the planar disilene structure is predicted to be the most stable. It may be very slightly twisted at high accuracy CI, but it is much lower in energy than the silylsilylene structure. Vibrational frequencies and infra-red intensities are also reported. Theoretical photoelectron spectra of C and Si systems are presented and compared with experiment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00531921
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  • 15
    Electronic Resource
    Electronic Resource
    Ab initio calculation for properties of hydrogen bonded complexes H3N⋯HCN, HCN⋯HCN, HCN⋯HF, H2O⋯HF (1986)
    Springer
    Theoretical chemistry accounts 69 (1986), S. 491-503 
    Details
    ISSN: 1432-2234
    Keywords: Hydrogen ; Bonded complexes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Self Consistent Field calculations, using TZ+2P basis sets, are reported on four hydrogen-bonded complexes. Vibrational frequencies, rotational constants, infrared and raman intensities are compared with available experimental data. The basis set superposition error is shown not to be important for the calculation of these properties with this basis set.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526706
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  • 16
    Electronic Resource
    Electronic Resource
    Hydrogen-bonded complexes involving HF and HCl: the effects of electron correlation and anharmonicity (1987)
    Springer
    Theoretical chemistry accounts 71 (1987), S. 41-57 
    Details
    ISSN: 1432-2234
    Keywords: Hydrogen bonds ; Frequency shifts ; Anharmonicity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Calculations on the hydrogen-bonded complexes HCN⋯HF, H2O⋯HF, ClCN⋯HCl and (CH3)2O⋯HCl are reported. SCF harmonic values for the HF and HCl frequency shifts are in considerable disagreement with experiment, by as much as 100 cm−1. Calculations at the MP2 (harmonic) level yield improved agreement with experiment, reducing discrepancies to the order of 10 cm−1. We have also calculated all the cubic and quartic force constants for HCN⋯HF at the SCF level, so that the anharmonic constants, x rs can be evaluated. Although x 11 (v 1=H-F stretch) is large and negative, it is more than compensated by a positive x 16 (v 6=N⋯H-F bend), so that the anharmonic correction to v 1 is small and positive. The validity of these anharmonistudies is examined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00538480
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  • 17
    Electronic Resource
    Electronic Resource
    Disilene, silysilyene and their cations (1987)
    Springer
    Theoretical chemistry accounts 72 (1987), S. 69-69 
    Details
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00526556
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  • 18
    Electronic Resource
    Electronic Resource
    Solution of the Hartree-Fock problem by expansion onto nested basesAided by research grants to the Johns Hopkins University from the National Science Foundation (Grant No. GP-8702) and the National Institutes of Health (Grant No. 5 R01 GM14470). (1970)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 4 (1970), S. 245-255 
    Details
    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 for solving the Hartree-Fock problem in a finite basis set is derived, which permits each orbital to be expanded in a different basis. If the basis set for each orbital φi contains the basis functions for the preceding orbitals, φi-1, φi-2,… φ1, then the φi form an orthonormal set. One advantage over the standard Hartree-Fock method is that a different long range behavior for each orbital, as for example is required in the Hartree-Fock-Slater method, can be forced. A calculation on the ground state of beryllium is performed using the nested procedure. Very little energy is lost because of nesting, and the node in the 1s orbital disappears.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560040303
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  • 19
    Electronic Resource
    Electronic Resource
    Reactions involving CO2, H2O, and NH3: The formation of (i) carbamic acid, (ii) urea, and (iii) carbonic acid (1986)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 7 (1986), S. 283-293 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A complete understanding of the synthesis of urea, (NH2)2CO, from NH3, CO2, and H2O remains an unsolved problem. It is considered that the formation of the intermediate ammonium carbamate (or the equivalent carbamic acid) takes place through the interaction of neutral species, and that this part of the synthesis is open to ab initio computations. Such calculations are reported on the formation of carbamic acid from NH3, CO2, and H2O. We have also investigated the formation of carbonic acid from CO2 and H2O showing that the six-membered ring transition state is non-planar, in contradiction with earlier reported calculations.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540070306
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