Showing papers on "Ab initio quantum chemistry methods published in 1974"

Perturbative ab initio calculations of intermolecular energies. I. Method

Abstract: Starting from a knowledge of approximate wave functions of the isolated molecules (or atoms) A and B, a method is proposed to build up a zeroth-order ground state and excited configurations for the complex AB in which the molecular orbitals keep their local significance. The standard Rayleigh-Schrodinger perturbation in this basis provides a decomposition of the zeroth-order interaction energy as a sum of the electrostatic and repulsion energy. In the second order, it is possible to identify the classical dispersion and polarization forces (modified by a term of order S2) and two additional contributions which are linked to the exchange possibility. The intramolecular correlation component is taken into account and compared with the correlation on the isolated molecules. It is moreover suggested that since we work in a rather limited basis set, the perturbed energy of AB must be compared with the unperturbed energies of A and B calculated in a basis including the vacant orbitals on their respective partner. Finally a possibility for going beyond the second order is described.

Theoretical study of curve crossing: ab initio calculations on the four lowest 1Σ+ states of LiF

Abstract: The potential curves of the four lowest 1Σ+ states of LiF have been studied by ab initio configuration interaction methods, using a contracted Gaussian basis set of better than double‐ζ quality, augmented with diffuse basis functions on the fluorine atom. The choice of orbitals and selection of configurations were carried out with the objective of obtaining a balanced treatment of the four states and of the different regions of the potential curves. ``Generalized valence‐bond'' orbitals, obtained from ground‐state MCSCF calculations, together with ``improved virtual orbitals,'' were found suitable for the construction of the configuration functions. The latter were energy selected from all single and double excitations relative to a set of 12 ``reference configurations,'' chosen to include all contributions found important for any of the four states at any internuclear distance. The expected avoided crossing between the lowest covalent and ionic structures is found at an internuclear distance of about 11 ...

Photoemission spectroscopy using synchrotron radiation. I. Overviews of valence-band structure for Ge, GaAs, GaP, InSb, ZnSe, CdTe, and Agl

Abstract: We determine bandwidths and critical-point positions with respect to the valence-band edge for the valence bands of several extensively studied semiconductors (with an accuracy of about 0.3 eV) by using photoemission densities of states derived from photoemission spectra obtained in the 24-78-eV photon energy range. These photoemission spectra were obtained using synchrotron radiation from an electron storage ring; a double-pass, electrostatic, electron energy analyzer; and samples cleaved and measured in situ in ultrahigh vacuum. We give a detailed description of the data-reduction techniques by which electronic state densities and certain valence-band feature positions are determined from photoemission spectra. This description includes a discussion of the effect of various phenomena such as optical-transition-probability variations and Auger-emission peaks. Tables are presented which compare our valence-band-position values with the results of both empirical calculations fit to optical data as well as ab initio calculations. We find systematic and significant differences between experiment and calculations fit only to optical data, these differences increasing with crystal ionicity, while ab initio calculations generally give a better fit to experiment.

Ab initio calculations of transition metal complexes

Abstract: The He(I) photoelectron spectra of the isoelectronic series Fe(CO)2(NO)2, Co(CO)3NO and Ni(CO)4 are reported and interpreted by means of ab initio SCF-MO calculations. For the nitrosyl complexes it is found that ionization potentials calculated assuming Koopmans' theorem are seriously in error due to the considerably greater orbital relaxation accompanying ionization from metal than from NO valence orbitals. When such allowance is made for orbital relaxation by performing restricted Hartree-Fock (RHF) calculations on the ionic states, the experimental spectra are accurately reproduced and the observed similarity of the spectra of all three molecules is explained.

Ab initio evaluation of correlation contributions to the true π‐electron Hamiltonian: Ethylene

Abstract: The first ab initio calculations of the correlation contributions to the matrix elements of the true effective π electron Hamiltonian are presented for ethylene. It is shown that there is an additional core ``correlation'' energy in the valence singlet excited (V) state that has no counterpart in the triplet (T) and ground (N) states. This correlation energy contributes to the reduction of the V–T separation and the contraction of the π*g orbital of the V state. Its neglect in ordinary Pariser‐Parr‐Pople theory, etc., results in a serious cancellation of errors in traditional semiempirical valence theories. When the correlation contributions are evaluated by second order perturbation theory, the final true parameters are much less dependent on the basis set (inclusion of diffuse orbitals) than are the original ``theoretical'' integrals.

Ab initio study of the X2Π and A2Σ+ states of OH. I. Potential curves and properties

Abstract: Accurate ab initio CI potential curves and molecular properties are presented for the X2Π and A2Σ+ states of OH. Results with known experimental values in parentheses are Re(X2Π) = 1.841(1.834) bohr, Re(A2Σ+) = 1.906(1.913) bohr, De(X2Π) = 4.43(4.63) eV, De(A2Σ+) = 2.29(2.53) eV, μ(OH,X2Π,ν=0) = 1.634(1.668) D, and μ(OD,A2Σ+,ν=0) = 1.861(1.72±0.10) D. Spectroscopic constants calculated from the theoretical potential curves are in satisfactory agreement with experimental results. Other molecular properties studied include quadrupole moments and the electric field gradient at the nuclei.

Geometries of the excited electronic states of HCN

Abstract: Ab initio quantum mechanical electronic structure calculations have been carried out for the ground state and 12 low‐lying (< 10 eV) excited states of HCN. A contracted Gaussian basis set of essentially double zeta quality was employed. A new theoretical approach, which should be widely applicable, was applied to the excited electronic states. First one selects a physically meaningful set of orbitals, which, hopefully, will be about equally suitable for all the electronic states of interest. After selecting a single configuration to describe each electronic state, configuration interaction is performed including all configurations differing by one orbital from any of the selected reference configurations. The method appears to be one of the simplest capable of treating several states of the same symmetry. The predicted geometries have been compared with the experimental results of Herzberg and Innes, as well as the appropriate Walsh diagram. The ab initio calculations and the Walsh diagram concur that Her...

Ab initio calculations of small hydrides including electron correlation: XII. the ions CH ? 5 and CH ? 5

Abstract: Five different structures of CH5+ and one structure of CH5− are calculated using a gaussian basis both in the SCF approximation and with the inclusion of electron correlation in the independent electron pair approximation (IEPA). While on SCF level the Csstructure of CH5+ has to lowest energy, the energy difference between the Csand C2vstructures becomes negligible if correlation is included. In contrast to this the approach of a proton to CH4 at large and intermediate distances is most favorable towards a corner of the CH4 tetrahedron which means a structure. The decomposition of CH5+ into CH3+ and H2 requires ∼20kcal/mol on SCF level and ∼40 kcal/mol if correlation is included.

Ab initio calculations on porphin

Abstract: Ab initioSCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin-restricted SCF calculations. The lowest state is found to have B1u symmetry with an ionization potential of 8.0 eV.

Charge‐conserving integral approximations for ab initio quantum chemistry

Abstract: Three charge‐conserving integral approximations are presented that either zero or economically calculate the less important integrals that arise in quantum chemical calculations on molecular systems and in solids. One of these is based upon the use of Gaussian basis functions. Results are presented for test calculations on glycine, glycylglycine, and diamond.

Perturbative ab initio calculations of intermolecular energies. II. The He...He problem

Abstract: The ground state energy of the He2 system is calculated according to the techniques described in Paper I around the van der Waals equilibrium, without assuming the constancy of the intraatomic correlation energies. The second-order results do not present an attractive region; the S2 decrease of the intraatomic correlation correlation corrections is larger than the attractive interatomic correlation corrections. The further orders reverse progressively the situation and finally give a qualitatively correct potential curve after the fifth order. Reaching almost exact solutions in the considered basis, one can demonstrate that the intermolecular calculation of the system A + B in the union of the basis for A and B involves an energy decrease which simply represents the effect of the extension of the basis set; and that a proper intermolecular calculation must compare the A + B energy with the A and B energies calculated in conveniently extended basis sets including the vacant MO's of the partner.

Relativistic and second order Z-dependent calculations for the 1s-2p transitions in Li I-like ions

Abstract: The transition arrays 1s22s-1s2s2p and 1s22p-1s2p2 in Li-like ions are calculated by means of relativistic Z-expansion combined with second order calculations of the electron electrostatic interaction. The results of the ab initio calculations for 6

Perturbative ab initio calculations of intermolecular energies. III. The water dimer

Abstract: The interaction energy between two water molecules A and B is calculated by the method described in Paper I [1], previously applied for the interaction between two helium atoms (Paper II) [2]. This interaction energy is obtained as the difference between the energies of the complex (A + B) and the monomers (A) and (B), obtained by a perturbation method. The results obtained with the perturbation developed up to the second order in a minimal atomic basis set are decomposed into classical contributions and contributions linked to the exchange possibility. Charge transfer contributions are important and the localized character of the hydrogen bond is examined. It is pointed out that the definition of the set of excited configurations for the calculation of the energies of the isolated monomers is important, especially when one tries to use a small atomic basis set. A similar effect in SCF-type calculations is evaluated. The contribution of higher orders is evaluated by the CIPSI method.

Valence electron studies with Gaussian‐based model potentials and Gaussian basis functions. III Applications to two‐valence‐electron systems composed of combinations of Li, Na, H, or their unipositive ions

Abstract: A previously developed simple valence‐only electronic structure theory based on atomic core model potentials and using flexible Gaussian valence basis functions is applied to the two‐valence electron systems Li2, Na2, NaLi, LiH, NaH, Li3+, Na3+, Li2Na+, LiNa2+, LiH2+, Li2H+, NaH2+, Na2H+, and H2, within the SCF MO model. Results for calculated equilibrium geometries and energy changes for certain chemical reactions are compared to the corresponding quantities from analogous all‐electron ab initio calculations. The model potential results are quite similar to those from the ab initio ones and are generally satisfactory for such a simple theory. There is a tendency toward slightly long internuclear distances (average deviation of all unique independent distances is +0.06 bohr) and slightly high energy of complex relative to separate constituents (average deviation of dissociation reaction energies is −2.1 kcal/mole) in these calculations relative to the all‐electron ones, the explanation of which will requi...

Ab initio calculations on some aspects of structure, bonding and reactivity of pyridine, phosphabenzene and arsabenzene

Abstract: Ab initio calculations have been performed on pyridine, phosphabenzene and arsabenzene and some of their ionized states. Changes in electronic structure as a function of the heteroatom have been investigated. Comparisons have been drawn between available experimental data from photoelectron spectroscopy studies (both ESCA and UPS) of the assignment of the higher occupied valence levels and also C1s core levels. The electronic reorganizations which accompany both core and valence ionizations have been studied. Limited investigations have been made of cross sections using prototype potential energy surfaces for protonation of the heteroatom.

The electronic structure of aromatic molecules. Non-empirical calculations on indole, benzofuran, benzothiophen, and related hydrocarbons

Abstract: Ab initio calculations in a gaussian orbital basis (LCGO) are reported for naphthalene, indene, styrene, indole, benzofuran, and benzothiophen. A bond energy analysis on small non-aromatic molecules yields energies for the classical Kekule structures; the resonance energies derived for the real molecules, in close agreement with figures based upon thermochemical data, are: naphthalene 357, indene 225, styrene 208, indole 308, benzofuran 232, benzothiophen 183, benzene 212 kJ mol–1. The calculated dipole moments are in reasonable agreement with experiment, and are separated into σ- and π-components. The high values of the π-moments, and the degree of scatter of the π-electrons into isolated pairs are related to aromatic character. Although the molecules are not isoelectronic, extensive correlation of the orbitals in the series can be achieved, and the HeI photoelectron spectra are interpreted in terms of the main groups of orbital energies.

Water-water pair interactions and the second virial coefficient of steam

Abstract: Recent ab initio calculations of the pair interaction between water molecules are claimed to be close to the Hartree-Fock limit. Second virial coefficient calculations for such a Hartree-Fock potential, together with a number of correlation energy corrections, are compared with results obtained from experiment. On the basis of this comparison it is shown that correlation energy terms are needed over the whole range of the interaction and it is not sufficient to directly add dispersion energy terms alone.

An Ab Initio Study of the Effect of Substituents on Protonation of the Carbonyl Group

Abstract: Ab initio calculations have been performed on a series of molecules HCOX, where X=CH3, NH2, OH, F, H, and also on the corresponding oxygen protonated cations. Rotational studies show the incoming proton to be trans to the substituent X except in formyl fluoride, where internal hydrogen bonding stabilizes the cis isomer. The geometries of the fluoro- and amino-substituted cations have been optimized along with that of oxygen protonated ketene. Proton affinities are found to be very dependent on substituents with the order of basicity being an order which closely follows the negative charge on the oxygen. Plots of the computed charge on the carbon atoms against the core binding energy are different for the cations and neutral compounds but can be made to fall on the same line by correcting the binding energy using "the charge potential model" for the K-shell electron which is removed.

Molecular energy levels of azines; ab initio calculations and correlation with photoelectron spectroscopy

Abstract: Non-empirical calculations using a standard gaussian atomic orbital basis (LCGO) have been carried out for known and unknown azines. The wavefunctions show pseudo D6h symmetry and have been correlated with those of benzene; this correlation has been used to interpret the combinations of ‘lone pair’ orbitals that occur. In the case of the tetrazines the splitting of the lone pair levels leads to extensive overlap with the CH region of energy so that mixing occurs and 1,2,4,5-tetrazine shows five orbitals which can be described as ‘lone pair orbitals’. The orbital energy ordering is effectively identical with that obtained from recent double ζ calculations on a few of the molecules; these energies correlate well with photoelectron spectra and obey the relationship IPobs= 0·790IPcalc. Correlations using CNDO/2, INDO, and the Extended Huckel Method (EHM) are less satisfactory both in grouping of the orbitals and absolute energies. The LCGO band assignments are generally in good agreement with those made by Lindholm et al. based upon the EHM. The thermodynamic stability of the azines with respect to fragmentation is discussed; the difference in energies between the classical alternating bond lengths and the observed ones for selected examples are in the region 6–10 kcal mol–1.