Abstract: An efficient procedure for third-order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self-energy expressions that are suitable for large molecules are empolyed. The outer-valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self-energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C with a 231 atomic orbital basis and for Zn(C5H5)2 with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time. © 1995 John Wiley & Sons, Inc.

Abstract: A two‐dimensional intermolecular potential energy surface for Ar–HF has been calculated using the many‐body symmetry‐adapted perturbation theory (SAPT). The H–F distance was kept constant at its equilibrium value. The interaction energies have been computed using an spdfg‐symmetry basis optimized for intermolecular interactions. In addition, the dispersion and induction energies have been calculated in a few progressively larger basis sets to determine the basis set convergence and validity of the asymptotic scaling of those components. Converged results for the dispersion energy have been obtained by using a large basis set containing spdfgh‐symmetry orbitals. The ab initio SAPT potential agrees well with the empirical H6(4,3,2) potential of Hutson [J. Chem. Phys. 96, 6752 (1992)], including a reasonably similar account of the anisotropy. It predicts an absolute minimum of −207.4 cm−1 for the linear Ar–HF geometry at an intermolecular separation of 6.53 bohr and a secondary minimum of −111.0 cm−1 for the linear Ar–FH geometry at an intermolecular separation of 6.36 bohr. The corresponding values for the H6(4,3,2) potential are −211.1 cm−1 at an intermolecular separation of 6.50 bohr and −108.8 cm−1 at an intermolecular separation of 6.38 bohr. Despite this agreement in the overall potentials, the individual components describing different physical effects are quite different in the SAPT and H6(4,3,2) potentials. The SAPT potential has been used to generate rovibrational levels of the complex which were compared to the levels predicted by H6(4,3,2) at the equilibrium separation. The agreement is excellent for stretch‐type states (to within 1 cm−1), while states corresponding to bending vibrations agree to a few cm−1. The latter discrepancies are consistent with the differences in anisotropies of the two potentials.

Abstract: A hybrid NVE Molecular Dynamics simulation of liquid water is presented using a coupled Density Functional/Molecular Mechanics hamiltonian. The quantum subsystem is a single water molecule described by means of a triple-zeta quality basis set with polarization orbitals on oxygen and hydrogen atoms. Non-local exchange-correlation corrections are included self-consistently. The classical system is constituted by 128 classical TIP3P water molecules. Results are in reasonable agreement with experimental data and in particular a good description of the solute polarization is obtained. Large fluctuations of the instantaneous value of the dipole moment of the quantum molecule are predicted.

Abstract: New ab initio potential energy surfaces (PES’s) are presented for the interaction of He with the NO radical in its ground (X 2Π) electronic state, determined within the coupled electron pair approximation (CEPA) with a large atomic orbital basis set. The dynamics of the collisions of NO with He are then investigated, in particular the coupling between nuclear motion (rotation and translation) and the internal electronic motion of the open‐shell partner. State‐to‐state integral and differential cross sections are calculated using full close coupling and coupled states methods. These cross sections are compared with the results of the two separate measurements at different initial collision energies, 508 and 1186 cm−1 (63 and 147 meV). Excellent agreement is obtained in both cases. Also comparisons with previous calculations, based on an earlier local density potential energy surface, are made at 508 and 2420 cm−1 (63 and 300 meV).

Abstract: The current experimental understanding of the interactions between hydrogen and defects in the MOS system, plus the theoretical progress to date toward explaining these observations are reviewed. One of the outstanding theoretical problems is the dissociation of hydrogen molecules in SiO 2 . A simple but fully quantum mechanical treatment of the dissociation of hydrogen molecules at silicon dangling orbitals in SiO 2 is presented. It is found that inclusion of quantum mechanical effects in the treatment of nuclear motion is fundamentally important to understanding the dissociation process. In the model calculations, it is estimated that the activation energy is 0.16 eV, in very good agreement with the experimental results of Li et al.

Abstract: Chemical concepts including global and orbital (or "divisional") electronegativity, hardness, softness, orbital hardness and softness kernel, and orbital Fukui index, for any given electronic system in a particular situation, are defined and relations among them are derived. Some results earlier obtained are used to illustrate uses of the formalism. Atomic parameters in various molecular circumstances are determined through semiempirical calculations as functions of corresponding atomic orbital population distributions. It is demonstrated how atomic energy changes such as ionization energy and electron affinity and transition energy can be calculated. Computed values agree well with experiment. The density-functional and L. C. Allen views of the electronegativity concept are reconciled. I. Introduction The density-functional theory of electronic structure is particularly appropriate for describing molecular electronic structure, because key quantities in density-functional theory are the familiar and useful concepts of electronegativity, atomic charge, hardness and softness, and frontier electrons.' The purpose of the present paper is to discuss atomic and atomic- orbital electronegativities and hardnesses in a manner appropri- ate when one is thinking of a molecule as a combination of atoms. Each atom in the molecule is an open system, in equilibrium with the other atoms with respect to interchange of electrons. The formulation we shall develop may, however, also be applied to the general case of a specific combination of functional groups and subgroups. For an atom or molecule having N electrons moving in a field v0 due to nuclei, the ground-state energy E(N,v) can be determined from a variational principle. The energy is a functional of the electron density, from the species in its ground state. We call -p or x the absolute ele~tronegativity.~ While the energy required to remove an electron is just I, equal weighting of I and A results in eq 3 because of the clear necessity to consider the relative tendencies of the two species to attract electrons. The energy required for A + B - A+ f B- is ZA - AB; the energy required for A -I- B - A- f B+ is IB - AA. These energies are equal if ZA + AA = ZB + AB. This was Mulliken's argument for eq 3.3 The sensitivity of p to N is itself a positive quantity of much interest,

Abstract: Triple-differential cross sections (TDCS) are measured in a coplanar symmetric energy sharing geometry for a neon target. The experiments were carried out for ionization of the 2p orbital between 500 and 65 eV incident energy and of the 2s orbital at 226.9 and 126.9 eV incident energy. The main difference compared to helium target is the apparition of a structure at 85 degrees for ionization from both orbitals. This structure, clearly seen at 65 eV for the 2p shell, is enhanced as the incident energy decreases. Distorted-wave Born approximation (DWBA) calculations including post-collisional interactions (PCI) and polarization are compared with experimental data. This theoretical model is found to give quite a good fit above 200 eV incident energy for ionization of the 2p shell and from 126.9 to 226.9 eV for ionization of the 2s. Below 100 eV, some disagreement between theory and experiment is observed. Calculations in this energy region, nevertheless, enable the effects of target orbital polarization to be identified.

Abstract: The Inelastic Scattering Cross-Section. There is a clear distinction between energy loss to inner-shell electrons and to valence or conduction electrons. The reason for the different treatment of these two processes is essentially that, in the former case, the initial state has a sharp energy while in the latter, there is a range of energy within the valence or conduction band. Orbitals of inner shell electrons show almost no overlap between neighbouring sites, so the exchange integral is negligibly small, leading to an extremely small bandwidth. The K-band in Na has a width of 2 × 10-19 eV, and a K-electron in Na jumps roughly once a week to a neighbouring site [3.1]. Those electrons — loyal to their atoms — are well described within an atomic model, which means that energy loss to inner-shell electrons can be treated within atomic theory. (Strictly speaking, this is true only for final states far above the Fermi level. When the excited inner shell electron, after interaction with the fast beam electron, occupies states slightly above the Fermi energy, the density of unoccupied states is mirrored as near edge structure, a typical solid state effect.)

Abstract: The electronic stopping power for bare particles and antiparticles (Zp=+or-1, +or-2, +or-3) incident on atomic hydrogen has been calculated by using a full classical and quantum-mechanical description of the energy-loss processes. For this purpose we have applied the classical-trajectory Monte-Carlo (CTMC) and the coupled-channel atomic orbital (AO) method. A comparison indicates that the classical results converge to the non-perturbative quantum-mechanical ones in the limit of strong projectile-induced perturbations, especially if the capture channels are of minor importance.

Abstract: The intermolecular chemical shift of a rare gas atom inside a zeolite cavity is calculated by ab initio analytical derivative theory using gauge‐including atomic orbitals (GIAO) at the Ar atom and the atoms of selected neutral clusters each of which is a 4‐, 6‐, or 8‐ring fragment of the zeolite cage. The Si, Al, O atoms and the charge‐balancing counterions (Na+, K+, Ca2+) of the clusters (from 24 to 52 atoms) are at coordinates taken from the refined single crystal x‐ray structure of the NaA, KA, and CaA zeolites. Terminating OH groups place the H atom at an appropriate O–H distance along the bond to the next Si or Al atom in the crystal. The chemical shift of the Ar atom located at various positions relative to the cluster is calculated using Boys–Bernardi counterpoise correction at each position. The dependence of the rare gas atom chemical shift on the Al/Si ratio of the clusters is investigated. The resulting shielding values are fitted to a pairwise additive form to elicit effective individual Ar–O,...

Abstract: Using these substructures, the molecular formula reduces to F1F2F3F4F5F6N2. MOLGEN obtains (after several steps not described in detail here) 2,337 structural formulas for that reduced molecular formula, and after expansion it constructs 8,916 isomers. We used the following restrictions: no triple bond and ring sizes between 5 and 6. Only 201 candidates remained from which an expert easily obtained the correct solution.

Abstract: A theory for excitons in X-ray spectra of conjugated molecules is presented. Due to the interaction with the core hole the lowest unoccupied pi level splits from other empty levels. The energy position of this state is defined by the renormalized Coulomb interaction with the core hole and by the width of the unoccupied pi band. The ratio of these two quantities determine whether the X-ray state is excitonic or not. It is shown that the conditions for excitonic character are qualitatively different between occupied levels (X-ray emission) and unoccupied levels (X-ray absorption). From the specified normalization we find that only half an electron density of the unoccupied molecular orbital can be localized to the atom with the core hole, while in the X-ray emission case the electron density of the lowest occupied pi orbital can be totally localized. The degree of localization of the lowest unoccupied pi level decreases when the length of the molecule increases. Contrary to lowest occupied and unoccupied molecular orbitals the electron density of higher occupied (emission) and higher unoccupied (absorption) pi orbitals is pushed out from the core excited atom for sufficiently large Coulomb interactions between core hole and valence shell. Strong site and size dependences of the intensity and frequency of excitonic peaks in X-ray absorption spectra are predicted, the site dependency being alternant for conjugated molecules. The model predicts strong depression of the absorption intensity for the pi * levels along the polyene series in good accord with ab initio data.

Abstract: The structure of the ground and lowest two excited states of H2NO have been determined in large scale configuration interaction calculations using a multiconfiguration self‐consistent description of the molecular orbitals. These treatments are based on a systematic building of the correlation contribution which has been designed to account for the characteristics of the nitroxide group. This approach shows that the aminoxyl functional group is more than a three electron group shared by two atoms, but, in fact, a nine electron entity. Our best estimate of the geometry of the ground electronic state, obtained after second‐order configuration interaction using a large basis of atomic natural orbitals, is pyramidal. However, since the potential depth between 0° and 40° is lower or of the same order of magnitude as the estimated inversion frequency, the conclusion that this molecule behaves like a planar system is totally justified. The structure of the excited (n−π*) and (π−π*) states have been determined and the transitions energies are in accordance with the experimental results on the highly substituted stable nitroxide radicals.

Abstract: We have measured the V 2 p 3 2 and O 1st X-ray absorption spectra of single crystal V2O5 and V6O13 and compared to linear combinations of atomic orbitals (LCAO) density of states (DOS) calculations. The spectra change dramatically with incident angle. The use of polarized light and a single crystal limits the number of transitions possible, revealing spectral features that cannot be resolved on polycrystals (angle-integrated). The measured O 1s and V 2 p 3 2 spectra agree with the projected unoccupied O 2p and V 3d DOS, respectively, indicating that atomic and solid state effects, including VO hybridization, must be included to describe adequately the spectra.

Abstract: Discrete variational Xα(DVXα) calculations have been carried out for the eight-co-ordinated vanadium-(IV) and -(V) complexes [V(hida)2]2–/1–(H3hida =N-hydroxyiminodiacetic acid). These complexes are minimal models for the corresponding oxidation states of amavadin from Amanita muscaria which, as isolated, consists of an approximately equimolar mixture of the anions [Δ-V{(S,S)-hidpa}2]2– and [Λ-V{(S,S)-hidpa}2]2–(H3hidpa =N-hydroxy-2,2′-iminodipropionic acid). The electronic structure and the nature of the metal-ligand bonding of these vanadium-(IV) and -(V) centres is described. The ordering of the d orbitals and their composition is closely related to that established for the oxovanadium(IV) moiety which demonstrates why, for many years, amavadin was mistakenly thought to contain a VO2+ centre.

Abstract: We have studied a mechanism for the stabilization of monolayer graphite on Si-terminated SiC(111) through a charge transfer which occurs between them by use of the first-principles molecular-orbital cluster method. The charge-transfer features change with atom configuration. At the commensurate site, the charge transfer hardly takes place, whereas at its neighbors a relatively large electronic charge transfers from the substrate to the overlayer. At the onset of the charge transfer an s orbital plays a crucial role such that the incommensurately (commensurately) sited carbon atom has a larger (smaller) s orbital component to give rise to larger (smaller) electronegativity.

Abstract: A prototypical description of the (Na19–Na)+ system is reported. The Na atom, in its ground and first two excited states, is treated by a one‐electron pseudopotential method. The cluster is first described in the spherical jellium background model (SBJM). A numerical Hartree–Fock approach is used to calculate the electronic wave function of the cluster in its ground state configuration. Singly excited Na19* states are obtained using an improved virtual orbital technique to allow for the distortions of the cluster electron cloud during the Na19–Na approach. The matrix of the electronic Hamiltonian in a (diabatic) basis of projected valence bond configuration state functions are determined with an effective model potential method. As a first model case, the (Na19–Na)+ system is treated holding the isolated positive background of the jellium cluster unchanged. This description pertains to rapid displacements of the atom relative to the cluster. As a second case, we consider distortion and reconstruction of t...

Abstract: The importance of the orbital radii, r l , obtained from the classical turning point of the valence electron wave function of angular momentum, I, in obtaining interatomic distances for all bonding situations is demonstrated. Single bond interatomic distances may be expressed in terms of a universal multiplicative constant of a core s orbital radius and another universal additive term which is close to the interatomic distance in the hydrogen molecule. These relationships are obtained from the dependence of the radii of positive or negative singly charged ionic species, CR + and CR - , on r l . The shortening of the distances in multiple bond systems or in systems involving transition metal d electron elements is described by a simple universal function, F s , associated with the number of unpaired valence electrons. A principle of maximum mechanical hardness based on minimization of bond distances is proposed to obtain correct distances in heteropolar MX bonds. The application of these rules to a large number of compounds with ionic, covalent, metallic, and nonbonded interactions yield interatomic distances which are within 2% of the observed distances. A brief discussion is made on the physical significance of the transferable length scales CR + and CR - in the context of discrimination of structure based on radius ratio and the requirement of a universal equilibrium chemical potential for transferability.

Abstract: Ab initio multireference CI calculations have been carried out for the HeN+ molecular ion in order to describe collision processes between its constituent neutral and ionized atoms. The accuracy of these calculations is evaluated by means of a comparison of results obtained at large internuclear separations with the corresponding asymptotic energies deduced from atomic spectral data. Energy values are computed for the eleven lowest He++N and He+N+ atomic limits and average discrepancies relative to the experimental excitation energies up to 110 000 cm−1 are found to lie in the 1000–3000 cm−1 range, of which only 200 cm−1 appears to be the fault of the configuration interaction (CI) technique itself, with the main portion of the error stemming from the choice of atomic orbital (AO) basis instead. The HeN+ X 3Σ− ground state is calculated to have a De value of only 1414 cm−1, but the excited 2 3Π state has a much larger value of 22 133 cm−1 by virtue of an avoided crossing with the lower state of this symme...