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Notes: To probe the decarboxylation process of methyl-ethyl-α pyridylacetic acid (MEPA), molecular orbital calculations on the optimized geometry, transition-state geometry, and intrinsic reaction coordinate were performed by the MNDO-PM3 method. The salient features of the optimized structure of MEPA are that the carboxyl anion is nearly on the plane of the pyridine ring (the dihedral angle of C8—C7—C2—N1 is 14.7°) and that the interatomic distance… is used for a noncovalent bond, such as N+ 1 … O-9. of O-9 … H1′ is 1.6 Å (exchange of electrons exists between their atoms). The transition-state geometry of the decarboxylation process has the following features: (1) the activation enthalpy is 6.0 kcal/mol, (2) the dihedral angle of C8—C7—C2—N1 is -50.2°, and (3) the interatomic distance of O-9—H1′ and C7—C8 increase by 111 and 124%, respectively, as compared with the optimized geometry. From the extreme beginning of the intrinsic decarboxylation process, the exchange of electrons between O-9 … H1′ begins to decrease. This decrease, which is considered to be induced by the rotation of C2—C7, seems to initiate the dissociation of C7—C8. © 1995 John Wiley & Sons, Inc.

Notes: A general method for solving the problems of spatially confined quantum mechanical systems is proposed. The method works within the framework of the model space approximation. In the case of atoms and molecules trapped into any-shape microscopic cavity (like molecular sieves or fullerenes), the method reduces to a simple modification of the commonly used basis-set quantum chemical calculations. The modification consists of a particular rotation and projection in the model space, leading to solutions better adapted to the boundary conditions of the spatial confinement than the functions that describe the free systems. To illustrate how this method works, it has been applied to the hydrogen atom confined in a spherical well, near a hard wall and confined in a cubic box. The results are also compared to the exact solutions. © 1995 John Wiley & Sons, Inc.

Notes: In the embedded cluster method, a cluster is embedded in a correction potential. This potential adds the long-range electrostatic forces of the crystal and subtracts the electrostatic potential of the boundary errors of the cluster. The correction potential is calculated from the wave function of the cluster and the crystal. So far, the method has only been used on a cluster having the same geometry and basis set as that of the crystal. Here, we calculated the adsorption energies of NH3 and NH4+ on an embedded zeolite cluster with different basis sets and modified geometries. We showed that a mixed basis set, a basis set having a large basis set on the atoms around the adsorption site and a minimal basis set on the atoms of the boundary of the cluster, yields adsorption energies close to that of the large basis set. The optimized geometries of the zeolite acidic site were almost equal for the nonembedded cluster, the embedded cluster, and the crystal. By combining the mixed basis set with a partial geometry optimization, an improved description of the adsorption process is obtained. © 1995 John Wiley & Sons, Inc.

Notes: Molecular structures of neutral and anionic beryllium hydrides have been investigated using ab initio calculations at the MP2 (geometries) and MP4 (energies)/6 - 31++G** levels. ZPE and (if possible) BSSE correction have been included. While BeH2- is not stable, the radical anion of the beryllium hydride dimer is significantly stabilized with respect to the neutral Be2H4 system. Moreover, this anion can appear in two isomeric forms with comparable stability. © 1995 John Wiley & Sons, Inc.

Notes: The nonlinear population analysis of pair densities introduced by one of us (R. B.) is reformulated using the geminal expansion of pair densities. The pair density in the geminal basis is idempotent and allows one to describe the bonding patterns as singlet-and tripletlike contributions while still conserving the statistical nature of the original formalism. The new approach is applied to the analysis of pair densities derived from semiempirical MNDO calculations. The resulting values of pair populations of several simple molecules are discussed. © 1995 John Wiley & Sons, Inc.

Notes: Using the connection between the evolution operator and the stationary value of the Lippmann-Schwinger functional, approximations to this operator are obtained using diagonal Padé approximants. A harmonic oscillator with a non-hermitean perturbation proportional to powers of the bosonic creation operator is considered and its evolution operator is evaluated. The poles of the spectral representation obtained by this method are compared to both: the ones of the usual perturbative expansion and those of the exact solution. Extensions to Hermitian Hamiltonians are discussed, involving the necessity of inverting more complex operators in the calculation of the Fourier transform. However, the approximation obtained by this procedure becomes exactly unitary. © 1995 John Wiley & Sons, Inc.

Notes: Structures and total energies of the cis-NO dimer, the trans-NO dimer, and the NO monomer were calculated by ab initio methods (UHF, UMP2, and MP3) and density functional theory methods (LSDA and BLYP) with different basis sets [from 3-21G* to 6-311++(3df,3pd)]. The system is especially hard to model because two NO molecules are weakly associated in a dimer that has a very long N—N bond. The results obtained by different methods are compared and the necessity of correlational methods for studying these systems is discussed. © 1995 John Wiley & Sons, Inc.

Notes: The lowest singlet and triplet states of s- and as-indacene have been studied by means of high-level ab initio MO and approximate density functional methods. Among the geometrical and energetical details discussed are the equilibrium structure of s-indacene (C2h or D2h), the structures and energies of the low-lying s-indacene triplet states, and the stability and geometry of the singlet and triplet states of as-indacene. It is shown that single-determinant-based methods, such as Hartree-Fock or MP2, are not suited to properly describe these molecules. Instead, methods are required which explicitly take into account nondynamical and dynamical electron correlation. The results obtained by density functional theory-based methods compare very well with the most elaborate ab initio MO data and seem to provide an economical alternative even for molecules with a complicated electronic structure such as s- and as-indacene. © 1995 John Wiley & Sons, Inc.

Notes: In the resonating Hartree-Fock calculation, we have to calculate the matrix element between spin-projected nonorthogonal Slater determinants (S dets). The matrix element between coherently spin-rotated S dets are given by a linear transformation of the spin-projected ones. Using the inverse transformation, we get the projected matrix elements from the coherently spin-rotated ones. By appropriately choosing the angles of the spin rotation, the required computational time is considerably reduced. © 1995 John Wiley & Sons, Inc.

Notes: A separation of the dynamic linear response is developed, which distinguishes between the one- and two-electron contributions to the molecular response, by partitioning the RPA equation. The derivation of the partitioning is given in both an RPA, equation of motion, type approach and using the alternative, but equivalent, density matrix method. Three physically distinct contributions are obtained, called the direct, interaction, and back contributions. The direct term is composed entirely of one-electron effects, while the interaction and back terms account for the electron-interaction contributions to the response. Results for the dynamic dipole polarizability suggest that while the one-electron contribution is dominant in the zero-frequency limit, the two-electron contribution becomes increasingly important as the frequency of the perturbation increases. This implies that approximation of the linear response by only one-electron contributions is acceptable for the static case, but is less relevant for the dynamic case. The ramifications of this observation, for the scaling of sum-over-states-type calculations of large molecular systems, is briefly discussed, as is the application of our partitioning method to the higher polarizabilities. © 1995 John Wiley & Sons, Inc.

Notes: On the basis of the recently proposed accurate calculation scheme of the inner-sphere reorganization energies (RE) of the reactants in gas-phase electron-transfer xprocesses, the inner-sphere RE values for the AH + AH+ (A = Mg, Al, Si, P, S, Cl) self-exchange systems are calculated in terms of an ab initio Hartree-Fock self-consistent-field MO method at different basis-set levels (6-31G**, 6-31 +G**, DZ, and DZP). The structural parameters involved are also determined via the perturbation theory and the Dunham expansion of the Morse function and compared with the experimental values. Dissociation energies are corrected by electron correlation at the MP2/6-31G* level. Results of the inner-sphere REs obtained from different models via ab initio calculations for these systems discussed here are in full agreement with the corresponding experimental data. © 1995 John Wiley & Sons, Inc.

Notes: We apply the CFHH-GLF method, a modified version of our HH-GLF method, to directly solve the three-body Schrödinger equation for a set of He-like systems, including H-, He, Li+, Be2+, and B3+. Correlation functions with no adjustable parameters are determined from the cusp condition of the wave function. Our calculational results exhibit very fast and good convergence with hyperspherical harmonics (HH) and a generalized Laguerre function (GLF) and substantial improvement over the HH-GLF method. With only 36 HH and 6 GLF, we obtained the ground-state energy of -2.90371, -7.27988, -13.6555, and -22.0308 au for He, Li+, Be2+, and B3+, respectively. This compares with -2.89361, -7.26131, -13.6253, and -21.9859 au, respectively, by the HH-GLF method and Pekeris' results of -2.90372, -7.27991, -13.6556, and -22.0310 au, respectively. So, the inclusion of 36 HH and 6 GLF has yielded the precision of a few parts in 106 for He, Li+, Be2+, and B3+. However, our calculational results for H- are not so good. We analyzed the cause of this kind of exception and improved our calculations in this respect by using a slightly different correlation function. We finally obtained the ground-state energy of -0.527754 au for H- with 36 HH and 15 GLF, which is very near Pekeris' result of -0.527751 au and of the same order of precision as those achieved for other He-like ions. © 1995 John Wiley & Sons, Inc.

Notes: Structures, heats of formation, and vibrational eigenvalues have been calculated by semiempirical AM1 quantum mechanical method for the reactants, products, and activated complexes of three Mannich reactions, namely, the reaction between potential Mannich reagent H2N+ = CH2Cl- and furan and the conventional Mannich reactions between ammonia, formaldehyde, and furan in neutral and acidic media. It has been found that the Mannich reaction involving the potential Mannich reagent occurs more quickly than do the conventional Mannich reactions in neutral medium; the Mannich reaction in acidic medium proceeds most quickly. The results have been discussed in terms of the orbital match and static electric interaction between some atoms of reactant complexes. © 1995 John Wiley & Sons, Inc.

Notes: The half-projected Hartree-Fock wave function (HPHF) is one of simplest models for introducing some electronic correlation effects. In this model, the wave function is built up with only two Slater determinants. This simple form suggests its application for the direct determination of singlet excited states. On the other hand, because the HPHF model does not mix singlet and triplet states with Ms = 0, it can be used for determining independently singlet and triplet states without any mutual contamination. In the present work, we applied this model to determine nine electronic states of the lithium molecule; one of them exhibits even the same symmetry of the fundamental one. For this purpose, the 6-311G(d) basis was used. Potential energy curves were determined and some spectroscopic constants derived. The numerical results were compared with the available experimental data, as well as with other theoretical values. © 1995 John Wiley & Sons, Inc.

Notes: Ab initio investigations have been carried out to account for the antitumor activity of flavone acetic acid (FAA) and related compounds. The hydrogen-bonding conformation was chosen for all the compounds and obtained through a restricted geometry optimization with the 3-21G basis set. Three key regions in the molecular electrostatic potential isosurfaces [V(r) ∼ -0.015 au] are found to be involved in the activity. Region 1 is the most important. Its existence implies the activity and its size determines the strength of the activity. Region 2 is another factor which can change the strength of the activity. Region 3 corresponds to the hydrogen-bonding effects which have been analyzed in detail, but its role is still not clear. Finally, basis-set effects on the molecular electrostatic potential have been briefly discussed. © 1995 John Wiley & Sons, Inc.

Notes: Convergence properties of the augmented Hessian (AH) method when searching for stationary points of an arbitrary fixed index are investigated. It is shown that the displacement vector of this method is proportional to one of the Hessian eigenvectors if the current point is far from a stationary one of the required index. A simple and reliable criterion for nearness of the current point to a stationary one of the desired index is proposed. The efficiency of a new one-dimensional optimization scheme that uses this criterion is studied. The case of coincidence of Hessian eigenvalues, which is a bottleneck of the standard AH method, is analyzed. A relation of the AH method to those by Poppinger and Wales is outlined. The correctness of the results obtained is illustrated on an example of a model surface. © 1995 John Wiley & Sons, Inc.

Notes: The derivation of localized atomic hybrids in the plane is discussed using their definition as simultaneous eigenfunctions of the x and y position operators, when these are represented by commuting matrices in a finite basis. The use of alias functions, as introduced in the preceding paper, to produce commuting matrices is described and illustrated. Full results are obtained for any regular polygon. The extension to three dimensions allows the sp2 trigonal, sp2d square, and sp2d2 pentagonal hybrids to be derived. The theory also allows the radial factors to differ so that several shells of hybrids can be found at once. The double trigonal configuration is described as an example. The implications for any calculation of a molecular dipole moment are discussed. © 1995 John Wiley & Sons, Inc.

Notes: Expressions for moments of spectral density distributions of a many-electron Hamiltonian defined in a finite-dimensional, antisymmetric, and spin-adapted model space (as, e.g., a full configuration interaction space) are derived. The moments are expressed in terms of combinations of two-electron integrals corresponding to a symmetric (a two-electron singlet) and antisymmetric (two-electron triplet) two-electron wave functions. A diagrammatic approach based on Hugenholtz-type diagrams and leading to a simple and universal classification scheme of the terms appearing in the expression for a specific moment is proposed. © 1995 John Wiley & Sons, Inc.

Notes: Relativistic basis sets for first-row atoms have been constructed by using the near-Hartree-Fock (nonrelativistic) eigenvectors calculated by Partridge. These bases generate results of near-Dirac-Hartree-Fock quality. Relativistic total and orbital energies, relativistic corrections to the total energy, and magnetic interaction energies for the first-row atoms have been presented. The smallest Gaussian expansions (13s8 p expansions) yield Dirac-Hartree-Fock total energies accurate through six significant digits, while the largest expansions (18s13p expansions) give these energies accurate through seven significant digits. These results are more accurate than some of the results reported earlier, particularly for the open-shell atoms, indicating that the basis employed is reasonably economical for relativistic calculations. © 1995 John Wiley & Sons, Inc.

Notes: A recently developed algorithm based on the coupled cluster ansatz for the generalized valence-bond perfect-pairing model is adapted for the NDDO class of semiempirical methods. It allows for the calculation of the nondynamic electron correlation and executes as rapidly as the semiempirical molecular orbital method. To assess the interplay of the nondynamic and dynamic electron correlation vs. parameterization, preliminary results are presented for rotation about a double bond, dissociations about single bonds, and the relative stabilities of biradical isomers. © 1995 John Wiley & Sons, Inc.

Notes: Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH3 with Li+, C≡N-, LiCN, and its isomer LiNC. The BSSE-corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.

Notes: A procedure of constructing a complete fragment orbital basis set is described in detail. It provides a complete fragment orbital basis for PMO analysis of the atomic interaction energy. Rt values, a ratio of the bonded atomic interaction energy to the nonbonded interaction energy arising from the peripheral atoms of the aromatic rings, indicate a decrease in the reactivity of the aromatic ring toward electrophilic attack in the sequence: pyrrole 〉 imidazole 〉 furan 〉 pyrazine 〉 pyrimidine 〉 aniline 〉 benzene 〉 pyridine 〉 p-acylaniline 〉 p-cyanoaniline 〉 p-nitroaniline 〉 m-cyanoaniline. © 1995 John Wiley & Sons, Inc.

Notes: The formula for the wave-function correction to the Hellmann-Feynman force provided by truncation of a basis set of orbitals centered at nuclei is reexamined. Correct application of a projector technique results in the validity of the Hellmann-Feynman theorem for finite basis sets. © 1995 John Wiley & Sons, Inc.

Notes: An analysis of the analytical properties of the first iterate of the iteration-variation approach applied to small Gaussian expansions is reported in the case of the hydrogen atom. The first iterates embody important qualitative improvements such as a cusp near the origin and better asymptotic decay at large values of r. Actual values of the properties depend on the quality of the initial functions in such a way that for very small Gaussian expansions the variation step is necessary. © 1995 John Wiley & Sons, Inc.

Notes: The nonempirical valence-bond method as well as the molecular orbital method are applied to investigate the structures of N2O2 and (CHO)2. Results show that in either molecule the σ-component tends toward a trans conformation, whereas, the π-component tends toward a cis conformation. Moreover, even if there are some long-range bonding overlaps in the cis structure, the bonding energy cannot counteract the increased Coulombic repulsive energy compared with the trans structure. In other words, for a cis conformation, there is a competition between the Coulombic destabilization and the π electronic stabilization. The latter is more preferable to a cis conformation of N2O2, while the Coulombic repulsive interaction should be responsible for the trans conformation of (CHO)2. The nonempirical vB calculations show that the π delocalization energy in N2O2 is negligible, while the value in (CHO)2 is about 4.5 kcal/mol. © 1995 John Wiley & Sons, Inc.

Notes: The photochemical decarboxylation mechanism of methacrylic acid near 249 nm was investigated with the unrestricted Hartree-Fock method. Our investigation shows that the methacrylic acid molecules obtained by photoexcitation are likely to undergo unimolecular decarboxylation either from its lowest 1nπ* state or, following internal conversion, from the ground state. These results support the photodecarboxylation mechanism proposed by Rosenfeld and Weiner. © 1995 John Wiley & Sons, Inc.

Notes: Careful study of radial spline basis-set convergence on the angular limits of the He 1s 2 state has been performed. Improvement of algorithm allows reaching the full ci limit in a large radial (79 spline functions) and angular (Imax=80) expansion. A variational energy of - 2.903724299061 au, with an error of 7.8 × 10-8 au is obtained, improving by over two orders of magnitude the previous available limit. Accurate angular momentum limits (1 × 10-8 au) are obtained up to l = 25. Analysis of the behavior of two-particle radial wave functions reveals a sharply peaked behavior which allows effective truncation of the two-electron radial basis for high angular momentum values. A reasonable estimate of computational resources required indicate that an accuracy below 1 × 10-8 au is presently feasible and about 1 × 10-9 au possible in the near future. © 1995 John Wiley & Sons, Inc.

Notes: A theoretical description of near-resonant charge exchange is presented for gas-surface collisions, based on the eikonal formalism, short wavelength approximation, and diabatic representation. The dynamics of the charge transfer is analyzed for the Na/W system. Accurate model diabatic potential surfaces are used to compute the charge exchange during scattering and desorption. The Pade method is applied to approximate the charge exchange probability. The derivations, developments, and comparisons of approximations of the charge-exchange probability are discussed. These approximations are useful in experimental surface analysis to estimate the integral value of diabatic coupling between the atomic and ionic states. The frequency of oscillation during the charge-exchange process is also discussed. Model diabatic potentials in analytic forms for the Na/W system are reported. © 1995 John Wiley & Sons, Inc.

Notes: This article is a brief review of the work formalism of electronic structure, its recent developments, and the results of its application to spherically symmetric and nonspherical density atoms. The formalism, which is founded in Schrödinger theory, is derived by physical arguments based on Coulomb's law. The fundamental quantity in the formalism is the pair-correlation density that constitutes the nonlocal quantum-mechanical source charge distribution giving rise to both a local potential representing electron correlations as well as the electron interaction energy. The potential is the work done to move an electron in the force field of the pair-correlation density and the energy the interaction energy between the electronic and pair-correlation densities. (For systems for which the curl of the force field may not vanish, the potential is obtained from the irrotational component of the field, the solenoidal component being neglected). The differential equation governing the system is a Sturm-Liouville equation, and as such, the exact wave function can in principle be obtained as an infinite linear combination of Slater determinants of the self-consistently determined spin-orbitals of the occupied and virtual states. The correctness of the interpretation for the local potential representing electron interaction is evidenced as follows: In the Pauli-correlated and central field approximations, ground-state energies of atoms (2 He -86Rn) lie within 50 ppm of those of Hartree-Fock theory, differing by less than 10 ppm for atoms with Z 〉 35. The densities thus generated clearly exhibit atomic shell structure and also satisfy the Kato-Steiner electron-nucleus cusp condition to 2 ppm. Another attribute of the formalism is that the asymptotic structure of the potential (when both Pauli and Coulomb correlations are considered) is that of the Pauli-correlated approximation. This is rigorously the case as shown for the He atom for which the potential vanishes in the classically forbidden region, the potential there being the exchange potential. As such, it is meaningful to compare the highest occupied eigenvalue of the differential equation in the Pauli-correlated approximation to experiment. A comparison for atoms and atomic ions of this eigenvalue to experimental ionization potentials and electron affinities show them to be consistently superior to the corresponding eigenvalue of Hartree-Fock theory. Transition energies determined from eigenvalue differences are also superior to those obtained from total energy calculations via Hartree-Fock theory when compared to experiment. Further, by considering the carbon atom in one of its degenerate ground states for which the curl of the field due to the Fermi hole does not vanish, it is shown that the solenoidal component of the field is negligible and two orders of magnitude smaller than is the irrotational component. Thus, the approximation of obtaining a path-independent potential for nonspherical density systems from the irrotational component of the field is accurate. Finally, Coulomb correlation effects can be incorporated within the work formalism in practice via the configuration interaction approximation. The self-consistent orbitals thus obtained explicitly incorporate the effects of both Pauli and Coulomb correlations in their structure because the source charge from which they are generated is a pair-correlation density. Furthermore, these orbitals possess the correct asymptotic structure since they are also generated by a potential that is local. The work formalism also provides a physical interpretation for the local potential representing electron correlations of Kohn-Sham density functional theory. Further, the exchange potential of the work formalism satisfies analytically two requisite conditions of the Kohn-Sham theory exchange potential. These are the scaling requirement and the sum rule relating the exchange energy to its functional derivative. The work formalism also leads to a deeper understanding of electron correlations in various approximations within Kohn-Sham theory. For example, it can be rigorously shown that the pair-correlation density in the local density approximation contains a term proportional to the gradient of the density. Thus, in contrast to the Kohn-Sham theory interpretation that electron correlations in this approximation are those of the uniform electron gas assumed valid locally, we learn that the nonuniformity of the electronic density is, in fact, explicitly accounted for by the approximation. This then explains the accuracy of the approximation. © 1995 John Wiley & Sons, Inc.

Notes: In this article, we justify the use of standard Kohn-Sham (KS) band structure theory for the calculation of the low-energy spectrum of metals and the density of states. For the higher spectrum, one can start the KS excited-state calculations by using as the first-order approximation the spin orbitals of the ground state. We next deal with the problem of degenerate states by using the subspace theory minimum principle. © 1995 John Wiley & Sons, Inc.

Notes: The coordinate scaling for the density matrix of ensembles of fractional occupied states of the density functional theory is explored. The ground-state adiabatic connection formula is extended to the ensemble exchange-correlation energy. © 1995 John Wiley & Sons, Inc.

Notes: The standard Kohn-Sham formulation of density functional theory (DFT) is limited, for practical reasons, to systems of less than about 50-100 atoms. The computational effort scales as Natα, where Nat is the number of atoms and 2 〈 α 〉 3. (By comparison, conventional configuration interaction methods are limited to 5-10 atom systems.) This article deals with the prospect of practical methods that scale linearly in Nat and may thus allow calculations for systems of 103-104 atoms. The physical reason (“near-sightedness”) for linear scaling is presented. Implementations of linear scaling DFT by the use of generalized Wannier functions or the one-particle density matrix are discussed. © 1995 John Wiley & Sons, Inc.

Notes: Attempts to put classical and quantum mechanics on an equal footing gave rise to establishing of many known phase-space representations of quantum mechanics. The class of self-dual representations occupies a distinguished position among them, as they could be endowed with a Hilbert space structure closely resembling the standard picture of quantum mechanics. Using a phase-space variational principle and the concept of a phase-space wave function, we established a phase-space version of the density functional theory parallel to the original Hohenberg-Kohn formalism and the constrained-search approach by Levy and Lieb. © 1995 John Wiley & Sons, Inc.

Notes: Photoionization cross sections are evaluated at the local density level avoiding any potential shape approximation by employing the Stieltjes imaging (ST) technique in conjunction with large (STO) basis LCAO calculations. The ST technique proves accurate in comparison with full continuum calculation on the noble gases ionization. Several choices for the final-state potentials, including different exchange-correlation potentials, are tested on the noble gases, as well as in H2O and CO. Finally, several molecules are investigated employing the transition-state xα potential and the results compared with available experimental data and previous calculations. The accuracy of the proposed approach appears quit comparable to the ab initio static-exchange level, removing spurious resonances associated to the muffin-tin approximation, and should prove useful in the analysis and interpretation of cross-section data for large systems, where it is shown that basis-set requirements become less stringent. The accuracy obtainable for the cross-section profiles allows close comparison with experimental data, which, although showing good general agreement for the shape of the cross-section profile, indicates the need of a further refinement of the theoretical model. © 1995 John Wiley & Sons, Inc.

Notes: The geometries of the two most important tautomeric forms of adenine and the corresponding methyladenines are investigated by means of quantum chemical calculations at different level of sophistication, ranging from semiempirical methods to correlation corrected ab initio methods at second-order Møller-Plesset perturbation theory level (MP2). The relative stability of the N(7)H ↔ N(9)H tautomeric forms of adenine are investigated with highly correlation corrected methods, MP3 and MP4. The relative stability is also corrected for solvent interactions and compared with experimental information. N(9)H-adenine is predicted to be the most stable tautomer in both vacuum and in solution. The relative stability is predicted to be between 24.5 and 35.0 kJ/mol in vacuum depending on computation method. In water solution N(7)H-adenine is stabilized more by the solvent, and the corresponding relative energies were found to be between 4.9 and 10.2 kJ/mol. We also found that correlation effects are essential to describe the ground state geometry with a high accuracy. The geometries predicted by semiempirical methods and ab initio calculations without correlation correction show large deviations in some parts of the molecule compared to the MP2 results as well as compared with experimental geometries. © 1995 John Wiley & Sons, Inc.

Notes: Ab Initio crystal orbital calculations on three-dimensional crystals/crystallohydrates of a number of diprotonated mononucleotides have been performed using the CRYSTAL92 routine package. The present results help to gain a deeper insight into the physical mechanisms of nucleic acid semiconductivity, as well as into the essence of intermolecular interactions and solvent effects in solid-state samples of nucleic acids. © 1995 John Wiley & Sons, Inc.

Notes: The average local electrostatic potential function, V(r)/ ρ(r), is calculated for 87 atoms, Li-Ac, in the ground state using the nonrelativistic average-over-configuration numerical Hartree-Fock density. It is found empirically that in a given atom the shell boundaries are expressed as the successively increasing maxima in V(r)/ ρ(r) and the outermost maximum presents good approximate estimates of the core-valence separation in atoms. The likeness in behavior of V(r)/ ρ(r) at each shell boundary with the maximum hardness principle is discussed. The single-exponent-fit parameters for the electron density in the valency region are provided for all atoms. © 1995 John Wiley & Sons, Inc.

Notes: We present here a perturbative analysis of the coupled cluster response method for molecular static properties with Euler and extended coupled cluster functionals under cubic truncation. Comparative analysis is meant to cater to both pedagogical and practical interests. Comprehensive tables for energy-derivative expressions and equations at the stationary point are presented. © 1995 John Wiley & Sons, Inc.

Notes: The ground-state potential energy curve of the F2 molecule as well as spectroscopic constants were calculated by means of the second-order quasi-degenerate many-body perturbation theory within a full (eight) valence orbital space using a DZP basis set. The problem encountered with a large number of valence electrons is avoided by a proper redefinition of the Fermi vacuum. A comparison with other related multireference techniques is also provided. © 1995 John Wiley & Sons, Inc.

Notes: To theoretically investigate the complexation and decomplexation processes in the reaction C60 + He ⇌ (He@C60), four possible reaction paths are assumed, so that while He approaches and penetrates the C60 cage (a) a pentagon, (b) a hexagon, (c) a short bond, or (d) a long bond will be expanded on its original sphere or plane to form a window. The computation is performed by the quantum chemical method EHMO/ASED. The results show that the probability of completing this reaction in terms of the tunnel effect can be neglected and the reaction is completed by overcoming a potential barrier of the reaction. It is easily completed by opening a planar window than by opening a shperical window. The probability through reaction path (b) with a barrier of 1247.94 kJ/mol is larger than that through reaction path (a) with a barrier of 1438.26 kJ/mol. The probability through reaction path (d) is the largest with a barrier of 1076.78 kJ/mol, when the planar expansion forming a window of a 9-membered ring is at the optimized value of 0.40 å. When He deviates the center and approaches the C60 cage, there will be a charge distribution on C60 with changing in size and even in sign along the longitude corresponding to the symmetry axis, but the absolute value gradually decreases. This situation is similar to the charge transfer on carbons in sraight-chain polyene. © 1995 John Wiley & Sons, Inc.

Notes: A brief review is given of the historical development of the treatment of the correlation problem in solving the Schrödinger equation in modern quantum theory from the early 1930s up to now. The correlation energy for a specific state of a system is defined as the difference Ecorr = E - EHF between the exact eigenvalue E and the Hartree-Fock energy EHF of the same Hamiltonian for the state under consideration. From the concepts of the “Different Orbitals for Different Spins” (DODS) and the “Alternant Molecular Orbital” (AMO) methods introduced in the 1950s, the study goes to the use of transition formulas - instead of expectation values - for the Hamiltonian and the possibility to express the correlation energy exactly in terms of “double excitations” or pair functions. The main emphasis is then put on the concept of wave and reaction operators and the formulation of the results of infinite-order perturbation theory in terms of such operators. The partitioning technique offers a simple way to derive these operators and to explore the resolvent or propagator methods in greater detail, and it replaces the original Schrödinger equation with its degeneracies and multiple roots by a reduced characteristic equation having only single roots, which is often a great simplification. Special attention is given the treatment of Schrödinger's perturbation theory in view of the importance of the linked-cluster theorem in the applications. In the study of the splitting of degenerate levels due to a perturbation, the use of the multidimensional partitioning technique utilizing the concept of an energy-independent wave operator is also discussed. Some developments in the coupled-cluster methods are further briefly reviewed. Since all quantum mechanical calculations going beyond the Hartree-Fock method imply a certain treatment of the correlation problem, a brief survey is also given of the progress in computational quantum theory, particularly in current quantum chemistry. © 1995 John Wiley & Sons, Inc.

Notes: An alternative ansatz for r12-MR-CI is given. The final formulas are much more easily obtained than was possible previously. The numerical performance is demonstrated with a model calculation of the ground state of helium using three different Gaussian basis sets (5s, 10s, 16s). With the two largest basis sets, the energies obtained with the new formulas, the formulas of Part I of this series of articles, and the formulas of the original formulation agree within 2 μEH. Since the new ansatz is much easier to handle mathematically, we recommend its usage for future work. © 1995 John Wiley & Sons, Inc.

Notes: It is possible to derive energy derivatives for nonvariational (e.g., coupled-cluster) methods invoking the generalized Hellmann-Feynman theorem. In such a procedure, one constructs a functional which, besides the usual wave-function parameters, contains new ones. One set of stationary conditions will reproduce exactly the original equations of the method, while the others will determine the value of the new parameters. We applied this straightforward procedure to derive analytic energy derivatives for several coupled-cluster (CC) methods applicable to excited states such as the Hilbert-space CC method, two-determinetal (TD) CC method, Fock-space CC method, and equation-of-motion-CC (EOM-CC) method. Finally, we compared the computational requirements for the different methods. © 1995 John Wiley & Sons, Inc.

Notes: We performed ab initio self-consistent calculations for the five alkali-earth sulfides - BeS, MgS, CaS, SrS, and BaS - by a method which allows the direct calculation of the ground-state electron density without a preliminar determination of the wave functions and of the energy eigenvalues of the system. We report the results for the standard cohesive properties (equilibrium lattice parameters, dissociation energies, bulk moduli), a study of the relative stability of the 131 (NaCl), B2 (CsCl), and B3 (ZnS) phases, and of the behavior under pressure of these compounds (equation of state; pressure and change of volume associated to the structural phase transition B1 → B2 or B3 → B1). © 1995 John Wiley & Sons, Inc.

Notes: The collective electronic response of NanKn clusters has been studied for some model structures. In their low-temperature lowest-energy structure, those clusters have all the K atoms on the surface. The collective oscillation frequencies for clusters with the K atoms segregated to the surface are red-shifted with respect to the corresponding frequencies for isomers with a very similar underlying skeleton but with the Na atoms segregated to the surface. The collective frequency varies smoothly with respect to the degree of relative segregation. These results may be useful in the analysis of the collective response of large alloy clusters and microcrystals. © 1995 John Wiley & Sons, Inc.

Notes: The Becke exchange functional is used for calculation of properties of the jellium model using the slab geometry inside a box with the infinite potential barriers at the boundaries. We simplify semianalytical representation of matrix elements. We calculate the surface energies and work functions with self-consistent electron densities. For all densities (here, we give results in erg/cm2 for rs = 2.07 bohr, in comparison with the LSD approximation (-602)) and the uncorrected Pw GGA-II (-730), the Becke-II exchange only (-1212), and the Becke-II exchange with Perdew86 correlation (-830) [always close to Pw GGA-I (-814)] give smaller surface energies. The most important factor determining values of surface energies from different GGAS seems to be a form of a correlation potential. We also calculate the effect of finite slab thickness and the vacuum region thickness on the surface energy at the LSD level and indicate its importance in various jellium model calculations. © 1995 John Wiley & Sons, Inc.

Notes: Electrostatic and dispersion contributions of solute-solvent interactions have been considered in the self-consistent reaction field scheme and implemented in the LCGTO-DF (linear combination of Gaussian-type orbitals-density functional) method. Results for the tautomeric equilibrium of formamide-formamidic acid, for the cis-trans energy difference in dichlorodiammineplatinum(II), and for H2O—HF hydrogen-bond systems are in agreement with the available experimental and previous high-level ab initio data. The role of the dispersion energy is discussed for the different studied systems. © 1995 John Wiley & Sons, Inc.

Notes: Local (LSD) and nonlocal (NLSD) spin density calculations using different exchangecorrelation functionals have been performed to determine equilibrium geometries, harmonic vibrational frequencies (ωe), ionization potentials (IP), electron affinities (EA), dipole moments (μ), and singlet-triplet energy gaps (Δ EST) of SiH2, GeH2, and SnH2. Geometrical structures as well as vibrational frequencies are in agreement with the available experimental data and compare favorably with the most sophisticated postHartree-Fock computations performed until now. Both computed IPS (9.15 and 9.25 eV for SiH2 and GeH2, respectively) and EA of SiH2 (1.17 eV) compare favorably with experimental data (9.17, 9.21, and 1.2 eV). Accurate values are obtained also for singlet-triplet energy gaps. We report for the first time the electron affinities of all neutral systems and the spectroscopic constants of the cations and anions. © 1995 John Wiley & Sons, Inc.

Notes: Using the Full-Potential LMTO and LMTO-ASA methods we have carried out the calculations of electronic structure and chemical bonding for different types of oxygen vacancies ordering in δ-Bi2O3. Nonlinear changes of cohesive energy with the variation of vacancy concentration was found. The lattice with two vacancies per cell appears to be the most stable one with vacancies ordered along the 〈111〉 direction. The semiconducting gap formation is analyzed, and the suggestion is made that the gap corresponds to the structure with uniform distribution of oxygen atoms over all possible oxygen crystal lattice site. © 1995 John Wiley & Sons, Inc.

Notes: In this article, we discuss how to incorporate correlation aspects in the interaction of the electron with a “frozen” correlated target. We discuss the important aspects of correlation for e- + N2 scattering using configuration interaction, coupled cluster, and perturbative methods to describe the correlation in the target molecule. © 1995 John Wiley & Sons, Inc.

Notes: The recently formulated atomically oriented valence-universal theory (VU-CCSD/R) based on various complete model spaces is applied to the accurate description of the 2s2 1S, 2s3s 1S, 2p2 1S, and 3s2 1S states of Be. Two extensive 9s9p7d7f5g STO basis sets that differ by one s-type function are employed. Both standard and nonstandard solutions of the VU-CC equations are considered. The eight values of the ground-state energies obtained when considering various model spaces and basis sets were found to differ very little. The energies of the individual excited states disclosed a relatively weak dependence on the choice of model spaces defining the method (or, equivalently, on the fact whether they correspond to standard or nonstandard solutions). In turn, the energies of the excited states strongly depend on the STO basis set used in the calculations. To obtain a reliable description of both the ground state and excited states, well-balanced STO basis sets have to be employed. © 1995 John Wiley & Sons, Inc.

Notes: A detailed analysis of the performance of the QDPT CI method, which is based on the application of quasi-degenerate perturbation theory (QDPT) within the configuration interaction (CI) approach, is presented by comparing the QDPT CI results and those obtained from the diagonalization of the perturbative space. Calculations of core and valence excitation and ionization spectra of the CO molecule, which is a strongly correlated system, are carried out using a DZ basis set and the accuracy of the eigenvalues and eigenfunctions is checked through the comparison of excitation and ionization energies and intensities. Excellent agreement is obtained for all values, indicating the computational advantages of QDPT CI which is able to explore wide energy ranges without requiring the diagonalization of large spaces. © 1995 John Wiley & Sons, Inc.

Notes: We performed CAS-CI calculations on Li2 using a set of molecular orbitals (MO) optimized with a procedure that, in the case of highly symmetric molecules, permits extraction of a small set of MO out of a large set of atomic orbitals (AO). The dimension of the CAS-CI space was of about 12 million symmetry-adapted determinants. We determined some spectroscopic constants of Li2 with three different atomic basis sets of increasing quality. The values obtained with the largest atomic basis set are very close to the experimental results. © 1995 John Wiley & Sons, Inc.

Notes: A brief description of an extension of the time-dependent functional theory to coupled field systems such as electron-ion and electron-electromagnetic fields is given. This theory is further generalized to situations where the system is initially in thermodynamic equilibrium and is subjected to external time-dependent probes which may drive the system out of equilibrium. Such a generalization is based on the Liouvillean quantum field dynamics (LQFD). The above formalism provides a firm physical and mathematical basis for time-dependent simulation procedures such as those based on the Car-Parrinello approach by replacing the fictitious Lagrangian by a precise action principle. Several physical examples are given to illustrate the need for this formalism, where there is a necessity for treating all the field variables on equal footing and in a self-consistent way. A new alternate formalism of time-dependent functional theory based on a novel application of the Baym Φ-derivable method is also given in terms of LQFD. By choosing suitable processes to set up the (D-functional, our functional theory is transformed into a practical self-consistent approach to the quantum many-body equilibrium and transport problems, with built-in conservation laws for all the physically relevant variables). © 1995 John Wiley & Sons, Inc.

Notes: The analysis of the Zeeman effect in diatomic molecules gives rise to parameters that describe small but significant deviations of the molecular electronic g-factor from its free electron counterpart. In order to account for these deviations, correction terms to the standard Zeeman Hamiltonian have been evaluated in first-order perturbation theory. Results obtained from SCF and correlated multi-reference configuration interaction wavefunctions are presented for the molecules NO, O2, and SO. © 1995 John Wiley & Sons, Inc.

Notes: We present a density functional scheme for calculating the frequency-dependent linear response of superconductors. The central result is a set of integral equations determining the linear response of the normal and anomalous densities to external perturbations. Analytic solutions of these integral equations are obtained for homogeneous systems with separable effective interactions. For inhomogeneous superconductors, the formalism leads to a scheme for calculating the critical temperature without explicitly solving the gap equation. © 1995 John Wiley & Sons, Inc.

Notes: The ensemble N-representability problem for the k-th order reduced density matrix (k-RDM) as well as the problem of reconstruction of the N-particle system density matrices (N-DM) from a given k-RDM are studied. The spatial parts of the k-RDM expansion in terms of spin tensorial operators Θλk are represented using particular values (at specially chosen ) of the Radon transform of the N-DM spatial parts (or their sums) DNλ(x′ | x″) (here, is a d-plane in the n-space ∝n of x = (x′, x″)), with n = 6N, d = 3 (N - k), x′ ≡ (r′1,⃛, r′N), x′ ≡ (r1″,⃛, r″N ()). In this way, the problem is reduced to investigation of the properties of the functions . For a normalizable N - DM, it is proved that are bounded functions. The properties of implied by the N-DM permutational symmetry, Hermiticity, and positive definiteness are found. A formal procedure of reconstruction of all N-DM corresponding to a given k-RDM is proposed. © 1995 John Wiley & Sons, Inc.

Notes: The generalized valence bond method is extended for use in electronic structure calculations of two-atomic molecules made up of the heavy atoms. By applying a purely numerical Hartree-Fock (HF) or Dirac-Hartree-Fock (DHF) basis set, we were able to efficiently produce the potential curves for a large range of the interatomic distances. The configuration interaction and nonorthogonal basis sets are used to obtain an accurate solution for the realistic molecular systems. Adaptation for parallelism allows the simulation to be partitioned and carried out on the coupled multiprocessor network (SP-2) at the NIST. Calculations have been performed for Na2 and In2 quasi-molecules in excited states. © 1995 John Wiley & Sons, Inc.

Notes: Solvent effects on the thermodynamics and kinetics of some tautomerization reactions, chosen as the most representative of this wide class of reactions, are analyzed. Solvent effects have been modeled by the self-consistent reaction field approach that we have implemented in the ADF package. We investigated the structure and energetics of the saddle points for the formamide/formamidinic acid and 2-pyridone/2-hydroxypyridine tautomerizations. Our results, carried out both at local and gradient corrected levels, are in good agreement with previous post-Hartree-Fock computations in predicting the structure and the energy of the investigated saddle points. On the other hand, our computations show that solvent effects play a relevant role on the energetics of the reactions. In particular, the interconversion of 2-hydroxypyridine into 2-pyridone is sensibly favored in aqueous solution, being that the latter tautomers are stabilized by solvent interactions. This latter result is in agreement with experimental findings. © 1995 John Wiley & Sons, Inc.

Notes: The methodology and theoretical framework of Part I of this series of articles have been further developed to the setting of the Banach algebra B(B), of all bounded operators acting on a Banach space B. Using the above setting B(B), certain dynamical systems of chemical kinetic equations are illustrated in conjunction with Part I and with the analysis of more general systems, some of which will be made in Part III of this series of articles. The main theorem and its auxiliary theorem in the present article elucidate in a unifying manner the structure and the underlying pattern of the spectral symmetry of linear operators (acting on Banach spaces and Hilbert spaces) that are investigated in each of the research fields of dynamical systems and quantum chemistry involving the spectral symmetry of alternant hydrocarbons. © 1995 John Wiley & Sons, Inc.

Notes: Localized molecular orbitals, two- and three-center bond indices of 1,5-dicarba-closo-pentaborane (5) and 1,5-dicarba-closo-pentaborane (3) and their hydrocarbon analogs bicyclo [1,1,1]-pentane and [1,1,1] propellane, respectively, have been calculated using a suitable basis set. No previously reported three-center CBC bond has been found. However, a new three-center BCB bond has been observed in 1,5-dicarba-closo-pentaborane (5). © 1995 John Wiley & Sons, Inc.

Notes: We propose a new type of Gaussian basis sets for use in calculations of electron scattering by molecules. Instead of locating the basis-set functions on the atomic centers of the target molecule, we place primitive s-type Gaussians at the positions of a cubic lattice with a regular grid. The grid and the Gaussian exponent are fixed so as to give the best representation of the plane-wave function. Plane-wave functions and Green functions obtained by means of the cubic-grid basis set are tested graphically against exact functions and functions expressed by means of a conventional Gaussian basis set. © 1995 John Wiley & Sons, Inc.

Notes: In this article, we report an efficient computational procedure for electron scattering matrix elements in the previously developed cubic-grid Gaussian basis sets. The Green function matrix elements derived for the cubic-grid basis set are simpler and easier to calculate than are those available in the literature for conventional Gaussian basis sets. Special features of the cubic-grid basis sets may also be exploited for a very efficient computation of Coulomb and exchange integrals. Inelastic scattering amplitudes for vibrational excitations may be efficiently calcualted in the harmonic approximation by numerical differention of the T-matrix elements. © 1995 John Wiley & Sons, Inc.

Notes: Previously developed cubic-grid basis sets of various sizes were used for the calculation of cross sections for elastic and inelastic electron scattering by the He and Ne atoms and the H2O molecule by the T-matrix expansion method. The aim was to test the invariance of calculated cross sections with respect to the translation of the target molecule and to examine the effect of basis-set size on the results. We also present a simple procedure for accounting for long-range interactions from the part of space that lies outside the volume that contains the cubic-grid basis set. © 1995 John Wiley & Sons, Inc.

Notes: An analysis withing the one-dimensional Hubbard model of the 1E2g- type state in cyclic polyenes is reported. In terms of the resonance integral β, a small β analysis and asymptotic large β analysis have been carried out. Energy as a function of β has been calculated numerically, and a second state with 1E2g- symmetry has been found. Some missing information about the higher momentum state of this symmetry is provided. © 1995 John Wiley & Sons, Inc.

Notes: Some phase-fitted P-stable methods are developed here. These methods have an algebraic order two and four, an interval of periodicity (0, ∞), and a phase-lag of order inifinity. Applications of the new methods to an orbit equation and the resonance problem of the Schrödinger equation (this problem involves finding all values of the energy for which the phase shift δ (E) is equal to π/2) show that the new methods are much more efficient than are other well-known methods. Also, based on these two methods, a new variable-step technique is obtained. Application of the new variable-step scheme to the phase-shift problem of the radial Schrödinger equation and to close-coupled equations of the Schrödinger type shows that the new variable-step method is much more efficient than is the well-known method of Raptis and Cash. © 1995 John Wiley & Sons, Inc.

Notes: A recipe for the accurate calculation of eigenstates of quartic double-well oscillators is presented. The behavior of eigenenergies as a function of the coupling strength is also studied. The quality of the semiclassical (WKB) expression for eigenvalues is critically assessed in this context. The analysis also includes a discussion on the possibility of degeneracy in such one-dimensional systems. © 1995 John Wiley & Sons, Inc.

Notes: Master equations for propagators in quantum open systems and their spectral resolutions are derived. The Zwanzig partitioning scheme along the superoperator algebra are used to derive equations of motion for partitioned operators in a Liouville space. The reservoir influence on the dynamical evolution of operators is shown to lead explicitly to dissipative effects arising from memory terms in the evolution equations of such operators. It is also shown that spectral representations may be written in a self-consistent analytic way by means of the self-energy fields for transition energies of the system by taking into account the lack of the complete knowledge about the reservoir. A kinematic fluid interpretation of the resultant equations is given and an explicit form of the “collision” superoperator is obtained. Finally, a simple example to illustrate the determination of self-energy fields for the system-reservoir interaction corrections is given. © 1995 John Wiley & Sons, Inc.

Notes: Spherical Gaussians can be expressed as linear combinations of the appropriate Cartesian Gaussians. General expressions for the transformation coefficients are given. Values for the transformation coefficients are tabulated up to h-type functions. © 1995 John Wiley & Sons, Inc.

Notes: A new boundary treatment, a Hartree-Fock (HF) surface potential model, is proposed to deal with the surface effect in the solid-state cluster calculations using the LCAO-MO-SCF ab initio method. The surface potential arises from one or more atoms, which have no basis function and are added to the calculated cluster system. These atoms are placed in such sites so that the HF potential field of the calculated system should possess a point-group symmetry. The surface potential could be found by the corresponding HF potential using a symmetry operator. The fact that a rather symmetric electronic structure of the asymmetric cluster YBa2CuZn2O7 is obtained using the HF surface potential shows that the surface effect in the cluster calculations could be neutralized to a great extent. © 1995 John Wiley & Sons, Inc.

Notes: We study the hydrogen atom confined within spherical impenetrable walls. The potential in the box is the Coulombian potential and outside the box the potential is infinite. To compute the energy eigenvalues of the Schrödinger's equation, we use a method proposed few years ago by Campoy and Palma to solve free quantum systems. The energy eigenvalues are computed with great accuracy for different box sizes, in addition we also compute few position expectation values that are related with the hyperfine splitting, nuclear magnetic shielding, polarizability, and pressure. Our results are more accurate than the literature calculations. © 1995 John Wiley & Sons, Inc.

Notes: The structure and conformational stability of polythionylphosphazenes is investigated by modeling single polymer chains with small mimics. The model compounds are composed of repeat units of the corresponding polythionylphosphazenes. Two of the model compounds have hydrogens and two have chlorines as substituents on phosphorus atoms. The substituents on sulfur may be either fluorine or chlorine. Fully geometry-optimized structures and energies of the stable conformations involving rotations around the P—N bond near the sulfur are obtained using the density functional theory method. The structural and conformational analyses indicate that the rotation around the N—P bond leads to variations in the bond lengths, the SNP bond angle openings, as well as couplings between dihedral angles in different conformations in all model compounds. In addition, the conformational analysis suggests that the minima on the conformational potential energy surface in these compounds may be located in the vicinity of the following values of the NP—NS dihedral angle: -50°, 90° (or 60°), 180°, and 240°. It was found that the values of the conformational energy differences range between less than 1 to 5 kcal/mol. A comparison is made between the structural results obtained using the density functional theory and the ab initio molecular orbital theory for the global minimum structures. © 1995 John Wiley & Sons, Inc.

Notes: The Compton energy loss spectra of atomic targets exhibits usually a specific asymmetry due to the postcollisional electrostatic interaction between the remaining ion and the ejected electron. This asymmetry, or Compton defect, also exists in the scattering by molecules or solids. The first attempt to describe the Compton defect for the hydrogen molecule is proposed here. The Compton energy loss spectra depends particularly on the relative orientation of the momentum transferred by the incident particle and the molecular axis. A comparison between the respective magnitudes of Compton profiles and Compton defects was one of the aims of this article. The results emphasize the particular interest of a simultaneous interpretation of both symmetrical and antisymmetrical contributions in the study of molecular or solid-state electronic structures. © 1995 John Wiley & Sons, Inc.

Notes: We report the resonance energies (REs) of several fullerenes with 4-membered rings and their isomers with only 5- and 6-membered rings computed using the conjugated-circuit model [RE(CC)] and the topological resonance energy (TRE) model. Both aromaticity indices were normalized by dividing by the size of the considered fullerene [RE(CC)/e and TRE/e]. The results parallel the predictions by Gao and Herndon using the much more advanced SCF-UHF π-electron approach. A good linear correlation is found between the topologically defined indices [RE(CC)/e and TRE/e] and normalized SCF-UHF π-electron energy. © 1995 John Wiley & Sons, Inc.

Notes: The elongation method based on the molecular orbital (MO) theory, which enables us to extend a polymer with any molecular fragments theoretically, has recently been developed by our group. As the next step, we introduced an approach based on the crystal orbital (CO) theory into above treatment. In the present work, the elongation method was developed at the Hartree-Fock level with CNDO/2 parameters and applied to model systems composed of the cluster series of a polymer and the molecular stacking on a surface. In the cluster-series calculations, the hydrogen molecule [(H2)n], hydrogen fluoride [(HF)n], polyethylene, and polyacetylene were created successively to approximate their one-dimensional periodic polymers by using the MO-based elongation method. In the molecular-stacking models, we described the hypothetical surface of crystal as periodically arranged hydrogen molecules by the COs, and several hydrogen molecules were stacked up on the surface one after another with the elongation procedure. Furthermore, the lattice defect on surface in which a part of stacked layer is lacking was dealt with by our approach. We also treated carbon monoxide chemisorption on a periodic magnesium chain as a more realistic model. Results for these systems support the applicability of our method for nonperiodic interactions in one- and two-dimensional large systems. © 1995 John Wiley & Sons, Inc.

Notes: This article presents an application of the accurate calculation scheme proposed recently for the inner-sphere reorganization energies of molecules of the type AH2 (A = Al, Si, P, and S). A reasonable extension has been made. The inner-sphere reorganization energies for the title thermal electron self-exchange reactions are calculated in terms of ab initio MO self-consistent field method (HFSCF) at different basis-set levels (6-31G**, 6-31 + G**, DZ, and DZP) and the involved parameters are also determined. These calculated results have been calibrated by comparing optimized molecular geometrical parameters and corresponding energy properties with the experimental findings or other theoretical values. An approximation, in which the contribution from the bond length-bond angle to the potential energy surface is neglected, is adopted in constructing the calculation formulas via the function model. Its adequacy is discussed. Agreement among different calculation schemes is analyzed. © 1995 John Wiley & Sons, Inc.

Notes: Calculations are carried out using first-principles self-consistent local-density and nonlocal density theory of the electronic structure, the total energy, and the charge density of a variety of semiconducting and insulating compounds under hydrostatic and uniaxial pressure. For several cases, the transition pressure from one structure to another is determined as well as the pressure coefficients of the main band gaps. It is shown that several properties are calculated with adequate accuracy to be compared with experiment, so that values which have not yet been measured are trustworthy predictions. © 1995 John Wiley & Sons, Inc.

Notes: The extension of the PPP Hamiltonian for alternant cyclic polyenes to noninteger values of the pseudomomentum by imposing modified boundary conditions is discussed in detail. It is shown that a computer program for periodic boundary conditions can be easily adapted to the new boundary conditions. Full CI computations are carried out for some low-lying states of the PPP model of altemant cyclic polyenes (CH)N, (N even) at half-filling. The energy values obtained by using periodic (Bloch) and antiperiodic (Möbius) orbitals are used to perform energy extrapolations for N → ∞. © 1995 John Wiley & Sons, Inc.

Notes: This article reports the intrapair and interpair electron correlation energies of the radical NaCl2. The total interpair correlation energy dominates. Hence, the interpair electron correlation energy must be considered in building models for correcting computed correlation energies. The 6-311+G* basis set recovers only 32% of the total estimated correlation energy and 44% of this amount came from the core electrons. © 1995 John Wiley & Sons, Inc.

Notes: It is well known that hydrogen like atoms may be represented as Morse oscillators. Our aim in this article was to show how this result and the hypervirial theorem lead to a computational method with which the matrix elements for the Coulomb potential can be easily calculated. Also, a closed analytical expression for 〈n2l2|rk|n1l1〉 is obtained. © 1995 John Wiley & Sons, Inc.