Showing papers on "Ab initio quantum chemistry methods published in 1978"
123 citations
01 Mar 1978
TL;DR: In this paper, the MRD-CI was used for various states of the C 2 H 6 + ion in two different nuclear geometries and the results were compared with the experimentally observed ethane PES in the 14-25 eV region.
Abstract: Ab initio calculations of the MRD-CI type are reported for various states of the C 2 H 6 + ion in two different nuclear geometries and the results are compared with the experimentally observed ethane PES in the 14–25 eV region. The calculated vertical IP values for ionization out of the 1e u , 2a 2u and 2a 1g MO's respectively agree well with the locations of the three ionization maxima in this spectral range. The analogous findings for excitation out of the relaxed ionic ground state find several relatively low-lying species which occupy a 2pσ* MO in addition to states resulting from simple ionization of the neutral molecule. A number of Rydberg states are also calculated at the relaxed-ion geometry, from which results it is determined that the quantum defects for such species are from 0.40–0.45 units smaller than for their counterparts in neutral systems; these findings are clearly consistent with a decrease in the core penetrability of the Rydberg electron as the effective charge is increased to Z = 2.
114 citations
TL;DR: In this article, the configuration interaction (CI) calculations were performed for the /sup 3/B/sub 1/ and /sup 1/A/sub 2/ states of methylene and the separation was computed to be 10.6 kcal/mol.
Abstract: Configuration interaction (CI) calculations which include all single and double excitations have been performed for the /sup 3/B/sub 1/ and /sup 1/A/sub 1/ states of methylene. The basis sets used were of better than triple-zeta quality and included two sets of polarization functions. The separation is computed to be 10.6 kcal/mol. A trend is shown to exist from the carbon atom through CH to CH/sub 2/, which suggests that the actual splitting is less than 10 kcal/mol. This is in disagreement with the interpretation of the latest experimental data, but agrees with earlier experimental work.
99 citations
84 citations
TL;DR: In this article, the valence shell orbital energies for the various title compounds are assigned by a combination of He(I) and He(II) photoelectron spectra and ab initio MO calculations of double and single zeta quality.
61 citations
TL;DR: In this article, a procedure for optimization of molecular geometries is presented, combining ab initio calculations with vibrational molecular data from spectroscopy or empirical force fields, and theoretical cartesian forces are transformed to vibrational normal coordinate forces from which geometry increments are calculated.
49 citations
TL;DR: In this article, an exponential rational approximant functional form for the lowest energy diabatic potential energy surfaces of both A′ and A″ electronic symmetry for the Li+O−2 and Na+O2− systems was developed.
Abstract: The intermediates in both the inelastic and reactive collisions of alkali metals with molecular oxygen are the M+O−2 ion pairs. We develop here analytic forms for the lowest energy diabatic potential energy surfaces of both A′ and A″ electronic symmetry for the Li+O−2 and Na+O2− systems. An exponential–rational approximant functional form, which is found to provide an excellent description of the ionic alkali monoxide potential curves, is adapted to the alkali dioxide ion pairs. The degree of polarization of the O−2 charge distribution is modeled from available theoretical data for the homologous LiF2 system. With only a few variable parameters it is possibile to fit extremely well those features of the M+O−2 surfaces which are presently known either from experiments or ab initio calculations. The topologies of the A′ and A″ surfaces are substantially dissimilar. The lower energy surface (A″) for both LiO2 and NaO2 possesses a pronounced minimum in isoceles triangular geometry, as expected from the matrix...
45 citations
TL;DR: In this article, the 3nπ* states of glyoxal and methyl glyoxalin were investigated and the symmetry unconstrained ΔESCF transition energies, unlike the symmetry constrained results, were found to be very similar.
40 citations
TL;DR: In this article, the influence of an increase of the basis set, an increase in the number of neighbors and increase of density of k-points in the Brillouin zone on the calculated equilibrium geometries and band structures has been studied systematically.
Abstract: Ab initio calculations on the linear lithium chain, the linear lithium hydride chain and a beryllium hydride polymer have been performed using the crystal orbital method. The influence of an increase of the basis set, an increase in the number of neighbors and an increase of the density ofk-points in the Brillouin zone on the calculated equilibrium geometries and band structures has been studied systematically. A proper description of the unit cell and the interaction between neighboring cells turned out to be most important. Energy bands were found to be extremely sensitive to any variation in the basis set applied.
38 citations
37 citations
TL;DR: In this article, the absolute IR intensities of perdeuterio acetylene and acetylene have been measured using the Wilson-Wells-Penner-Weber method and the intensity data have been analyzed in terms of dipole moment derivatives, bond charge parameters and atomic polar tensors.
TL;DR: In this paper, the HeI and II photoelectron spectra obtained for 2,1-benzisothiazole and 2, 1, 3benzothiadiazole have been satisfactorily interpreted, by means of a combination of variations between compounds and ab initio MO calculations.
TL;DR: In this paper, the long range (electrostatic, dispersion, induction) and short range (exchange and penetration) interaction energy between ethylene molecules has been calculated by ab initio methods as a function of the molecular orientations and distances.
Abstract: The long range (electrostatic, dispersion, induction) and short range (exchange and penetration) interaction energy between ethylene molecules has been calculated by ab initio methods as a function of the molecular orientations and distances. The results, when fitted with an exp‐6‐1 atom–atom potential and used in a harmonic lattice dynamics calculation on the ethylene crystal, yield fair agreement with the experimental structure data, ir and Raman phonon frequencies. Although the fit with the atom–atom potential is reasonably good, some specific deviations from the ab initio results indicate the importance of the effects of chemical bonding on the intermolecular potential (leading to noncentral and nonpairwise additive atom–atom forces). The usual empirical atom–atom potentials are grossly corroborated, their main defect being the neglect or underestimate of electrostatic (quadrupole–quadrupole) interactions.
TL;DR: In this paper, an analysis of the intensity relations in the photoelectron spectrum of O2 for ionization of an electron from the 2σ u or 1π u molecular orbitals is presented.
Abstract: Ab initio calculations within the minimal basis of Slater-type orbitals are performed on the O2 + ion. Full-valence configuration interaction is carried out at seven internuclear separations between 1·0 A and 2·0 A. For eight observed bound states, the calculated ordering agrees with experiment, and the root mean square of the differences between the eight calculated and observed vertical excitation energies is 0·45 eV. An analysis of the intensity relations in the photoelectron spectrum of O2 for ionization of an electron from the 2σ u or 1π u molecular orbitals is presented. This leads to a better understanding of some characteristics of these bands and to a new assignment of a few observed peaks.
TL;DR: In this paper, the authors measured the reflectance spectra of ten crystalline chromate salts and compared them with the spectrum of the chromate ion in solution, and found that the increase in the intensity of the 23,000 cm − 1 band in crystalline compounds can be explained by the increasing polarizing effect of the cations and the appearance of new low-energy bands in case of highly covalent metal.
TL;DR: In this article, the Hartree-Fock perturbation theory for the Fock-Dirac density matrix has been applied to magnetic shielding in H2O, NH3, and CH4 molecules.
Abstract: Proton magnetic shielding constants in H2O, NH3, and CH4 molecules have been evaluated within the framework of coupled and uncoupled Hartree–Fock perturbation theories for the Fock–Dirac density matrix, employing several basis sets of gaugeless Gaussian functions. The results yielded by the largest basis sets are in good agreement with experiment, as well as other ab initio calculations. Some quantities, useful in testing the degree of gauge independence of computed screening constants and in evaluating this property for a gauge whatsoever, are also presented. Numerical tests demonstrate the basic soundness of the uncoupled approach to magnetic shielding proposed here, because of its higher accuracy with respect to other uncoupled schemes and its practicality for larger molecules.
TL;DR: In this article, the results of ab initio calculations on LiF, NaF, BeO, MgO, and their anions were reported, and the vertical electron affinities of BeO and LiF were 2.10 and 0.31 eV, respectively.
TL;DR: Using large Gaussian basis sets ab initio calculations on the molecules SN+, SN, and S2N2 have been performed at the SCF level as discussed by the authors, and electron correlation effects were studied for SN+ and S 2N2 by the PNO-CI and CEPA-PNO methods.
Abstract: Using large Gaussian basis sets ab initio calculations on the molecules SN+, SN, and S2N2 have been performed at the SCF level. Additionally, electron correlation effects were studied for SN+ and S2N2 by the PNO–CI and CEPA–PNO methods. The following equilibrium geometries were obtained: SN+, reSN =2.64 (SCF), 2.72 a.u. (CEPA); SN, reSN=2.78 a.u. (SCF); S2N2, D2h symmetry, reSN=3.07 (SCF), 3.16 a.u. (CEPA), &NSN=90.2° (SCF), 90.5° (CEPA). Complete harmonic force fields of S2N2 and the corresponding vibrational frequencies were computed. The assignments of the experimental ir and Raman spectra for S2N2 are discussed critically on the basis of the theoretical results.
TL;DR: In this paper, four different ab initio calculations were used to determine four sets of potential energies at a common set of geometries for the Li+H2 collision system.
Abstract: Four different ab initio calculations were used to determine four sets of potential energies at a common set of geometries for the Li+H2 collision system. The four calculations involved choices between a moderate and a large basis set and between the Hartree–Fock and the multiconfiguration‐self‐consistent field formalisms. Only the nonreactive ground electronic state surface was examined. An analytic functional form was least squares fit to each set of potential energies to produce four analytic potential energy surfaces. Classical trajectories at energies below the reaction threshold were run on each surface. The trajectories were analyzed to obtain the inelastic cross sections, differential cross sections, and average energy loss as a function of angle. The comparison of these collision observables from surface to surface was used to characterize the chemical accuracy of each surface. All four surfaces, from the least to the most accurate, produced significant amounts of rotational excitation.
TL;DR: In this paper, an energy-level diagram is built up which includes all the valence sextet states up to 60000 cm-1 and demonstrates the close resemblance of the electron structure of FeF to that of the Fe+ ion.
Abstract: For pt.I see ibid., vol.11, no.13, p.2281 (1978). Ab initio SCF-CI calculations of the electronic states of iron monofluoride have been performed with the aim of elucidating the ultraviolet spectrum of this radical. The bonding is ionic and the valence orbitals preserve their atomic character in the molecule. The ground state is found to be an inverted 6 Delta state associated with the (3dFe6( sigma pi 2 sigma 3) 4s sigma Fe2pF6( sigma 2 pi 4)) configuration. An energy-level diagram is built up which includes all the valence sextet states up to 60000 cm-1. It demonstrates the close resemblance of the electron structure of FeF to that of the Fe+ ion. The observed systems of FeF are shown to originate from 4s-4p transitions corresponding to raies ultimes of Fe+: the observed 6 Pi and 6 Phi states both belong to a single configuration, (3dFe6( sigma pi 2 delta 3)4p pi Fe 2pF6( sigma 2 pi 4)).
TL;DR: In this paper, small basis set SCF-LCAO-MO calculations are described for the molecules (C5H5)M(C7H7) with M = Ti, V, Cr and Cr.
TL;DR: The crystal structure of the NH4)2 [FeCl5(H2O] compound has been determined by single-crystal X-ray diffraction at 295 K and refined by full matrix least squares to a residual of 0.054 for 1662 'observed' reflections.
Abstract: The crystal structure of the title compound, (NH4)2 [FeCl5(H2O)], has been redetermined by single-crystal X-ray diffraction at 295 K and refined by full matrix least squares to a residual of 0.054 for 1662 'observed' reflections. Crystals are orthorhombic, Pnma, a 13.706(2), b 9.924(1), c 7.024(1)Ǻ Z 4. Fe-O is 2.110(4)Ǻ; Fe-Cl range between 2.323(1)Ǻ (trans to OH2) and 2.350(1)-2.390(1) Ǻ (cis), the latter distances being very dependent upon hydrogen-bonded interactions with nearby species.
TL;DR: In this article, the potential energy surfaces of the ground states of HCN and HSiN (HNSi) were derived using a combination of spectroscopic data and ab initio calculations.
Abstract: Analytic functions have been derived for the potential energy surfaces of the ground states of HCN (HNC) and HSiN(HNSi) using a combination of spectroscopic data and ab initio calculations. The potentials derived give no barrier to the approach of H to CN at any angle, but there is a barrier when H approaches SiN, this being larger at the Si end.
TL;DR: In this paper, the vertical electronic spectra of NO2, NO+2, and NO−2 were calculated at the NO2 ground state experimental equilibrium conformations using a basis set augmented by diffuse s and p functions.
Abstract: MCSCF/CI calculations of the vertical electronic spectra of NO2, NO+2, and NO−2 were carried out at the NO2 and NO−2 ground state experimental equilibrium conformations These calculations extend previous calculations of the same type in that polarization functions are included in the basis set and various limitations on the number of open shells and on the number of configurations in the MCSCF and CI calculations are removed Calculations were also carried out on NO−2 with a basis set augmented by diffuse s and p functions
TL;DR: In this article, it was shown that the use of experimental geometries in ab initio calculations of the dimerization energy of formic acid can lead to unrealistic results.
Abstract: It is shown that the use of experimental geometries in ab initio calculations of the dimerization energy of formic acid can lead to unrealistic results. This is caused by errors in the molecular energy difference between the free molecule and the distorted molecule as it occurs in the dimer. A method is suggested that overcomes this difficulty, and it leads to a value of about 3.5 kcal/mole for this molecular energy difference.
TL;DR: In this paper, the electronic coupling between the SI and SO surfaces, which can induce internal conversion, is calculated for this rearrangement and for the reaction coordinates leading directly to radical and molecular products.
Abstract: Ab initio calculations on the H2CO-HCOH rearrangement have been performed. The electronic coupling between the SI and SO surfaces, which can induce internal conversion, is calculated for this rearrangement and for the reaction coordinates leading directly to radical and molecular products. The coupling is calculated with true adiabatic Born-Oppenheimer functions, i.e., the wave functions and coupling integrals are explicitly calculated as functions of the nuclear geometry. The coupling for the hydroxycarbene rearrangement turns out to be the largest one. This indicates that the hydroxycarbene can serve as an intermediate state in the formaldehyde photochemistry. We also report calculations on the bimolecular H2COHCOH rearrangement; this interaction gives rise to a decrease of the energy barrier involved.
TL;DR: In this article, an ab initio calculation of the quasimolecular K-X-ray spectrum from the Pb + Pb system is presented, where both the Kd-hole amplitude and the radiation amplitude are calculated in first order perturbation theory.
TL;DR: In this paper, the deuterium bond of D5O+2 was treated by considering vibrations in two dimensions, and the vibrational energy levels, intensities and polarizabilities dependent on the electrical field strengths at the bond were calculated for a variety of bond lengths.
Abstract: On the basis of former ab initio calculations on H5O+2, the deuterium bond of D5O+2 is treated by considering vibrations in two dimensions. Vibrational energy levels, intensities and polarizabilities dependent on the electrical field strengths at the deuterium bond are calculated for a variety of bond lengths. From the absorption intensities continuous absorption spectra are calculated, taking into account distributions of electrical field strengths and O–O distances. These continua are compared with i.r. spectra of aqueous solutions of HCl or DCl and H2SO4 or D2SO4 in H2O or D2O, respectively. The calculated isotope factor of the intensity IνOH/IνOD, which is dependent on wavenumber, agrees with experimental data.