Showing papers on "Ab initio quantum chemistry methods published in 1980"

Generalized transition state theory calculations for the reactions D+H2 and H+D2 using an accurate potential energy surface: Explanation of the kinetic isotope effect

Abstract: Rate constants are calculated for the reactions D+H2→DH+H and H+D2→HD+D and compared to measured values. An accurate potential energy surface, based on the ab initio calculations of Liu and Siegbahn, was used. Rates were calculated using both conventional transition state theory and canonical variational theory. In the former, the generalized transition state dividing surface is located at the saddle point; in the latter it is located to maximize the generalized free energy of activation. We show that, in the absence of tunneling corrections, locating the generalized‐transition‐state dividing surface variationally has an important quantitative effect on the predicted rate constants for these systems and that, when tunneling is included, most of the effect of using a better dividing surface can be included in conventional transition state theory for these systems by using a consistent transmission coefficient for quantal scattering by the vibrationally adiabatic potential energy curve. Tunneling effects ar...

Vacuum ultraviolet photoelectron spectroscopy of transient species: Part 11. The NH2(X 2B1) radical

Abstract: The HeI photoelectron spectrum of the NH2(X 2 B 1) radical produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of NH2(X 2 B 1) to the X 3 B 1, 1 A 1 and 1 B 1 states of NH2 +. Measurement of the first two adiabatic ionization potentials of NH2 gave a value for the separation of the ground vibrational levels in NH2 +(X 3 B 1) and NH2 +(1 A 1) of 0·99 ±0·02 eV. Ionization energies of NH2 have been estimated using ab initio calculations which include the effects of electron correlation. These calculations indicate that two other bands arising from ionization of NH2, corresponding to formation of the 3 A 2 and 1 A 2 stages, should be observed in the HeI region but no clear evidence of these was obtained experimentally because of overlapping band problems. Spectra of the deuteriated radical were obtained and are consistent with these conclusions. Calculated and observed separations of the two lowest states of NH2 + are compared with those for the analogous ...

Integrated spatial electron populations in molecules: Application to simple molecules

Abstract: The electron projection function P(x, z) = ∫ ρ(x, y, z) dy is used to evaluate charge transfer and covalency in two series of molecules, LiX and CH3X (X = Li, BeH, BH2, CH3, NH2, OH, and F), with wavefunctions derived from STO-3G, 4-31G, and, in some cases, 6-31* ab initio calculations. The precision of the method and comparison with Mulliken populations analysis are described. Particular attention is given to CH3Li which by our criteria is wholly ionic.

Ab Initio Calculations: Methods and Applications in Chemistry

Abstract: 1. Introduction.- 2. Basis Set.- 2.A. Fundamental Concepts and General Description.- 2.B. Slater-Type Orbitals.- 2.C. Gaussian-Type Functions.- 2.D. Contracted Gaussian Basis Sets.- 2.E. Gaussian Expansion of Slater-Type Orbitals.- 2.F. Polarization Functions.- 2.G. Off-Centered Gaussian Functions.- 2.H. Comparison of Slater-Type and Contracted Gaussian Basis Sets.- 2.I. Remarks on the Selection of the Basis Set.- 3. SCF Calculations.- 3.A. Integrals over Slater-Type Orbitals.- 3.B. Integrals over Gaussian-Type Functions.- 3.C. Computer Time Saving in Evaluation of Integrals.- 3.D. Computer Time Saving in the SCF Procedure.- 4. Correlation Energy.- 4.A. Definition and Origin of the Correlation Energy.- 4.B. Conservation of the Correlation Energy.- 4.C. Empirical Calculations.- 4.D. Configuration Interaction.- 4.E. Independent Electron Pair Approximation.- 4.F. Cluster Expansion of the Wave Function.- 4.G. Many-Electron Theory of Sinanoglu.- 4.H. Coupled-Pair Many-Electron Theory.- 4.I. Coupled-Electron Pair Approximation.- 4.J. Perturbation Calculations.- 4.K. Numerical Treatment of Perturbation Expressions.- 4.L. Basis Set Dependence.- 4.M. Size Consistency.- 5. Applications.- 5.A. Molecular Geometries.- 5.B. Force Constants.- 5.C. Barriers to Internal Rotation and Inversion.- 5.D. Potential Curves.- 5.E. Thermochemistry.- 5.F. Chemical Reactivity.- 5.G. Ionization Potentials.- 5.H. Intermolecular Interactions.- 5.I. Solvation.- 5.J. Presence and Future.- Appendix A: Atomic Units.- Appendix B: Most Common Computer Programs.- References.

Ab initio and Gordon–Kim intermolecular potentials for two nitrogen molecules

Abstract: Both ab initio MO–LCAO–SCF and the electron‐gas (or Gordon–Kim) methods have been used to compute the intermolecular potential (Φ) of N2 molecules for seven different N2–N2 orientations. The ab initio calculations were carried out using a [4s3p] contracted Gaussian basis set with and without 3d polarization functions. The larger basis set provides adequate results for Φ≳0.002 hartree or intermolecular separations less than 6.5–7 bohr. We use a convenient analytic expression to represent the ab initio data in terms of the intermolecular distance and three angles defining the orientations of the two N2 molecules. The Gordon–Kim method with Rae’s self‐exchange correction yields Φ, which agrees reasonably well over a large repulsive range. However, a detailed comparison of the electron kinetic energy contributions shows a large difference between the ab initio and the Gordon–Kim calculations. Using the ab initio data we derive an atom–atom potential of the two N2 molecules. Although this expression does not a...

Theoretical studies of the collision-induced Raman spectrum of carbon dioxide

Abstract: Ab initio calculations have been performed on carbon dioxide to obtain the first derivatives of the dipole-quadrupole polarizability A and of the first hyperpolarizability β in the bending and asymmetric stretching modes. These derivatives are used to assess the relative contributions of A and β to the collision-induced Raman spectrum. It is concluded that most of the intensity arises from the mechanism involving the A tensor. Reasonable agreement is obtained with measured depolarization ratios and relative intensities for the v 2 and v 3 modes in compressed gaseous CO2.

Lattice dynamics of solid N2 with an ab initio intermolecular potential

Abstract: We have performed harmonic and self‐consistent phonon lattice dynamics calculations for α and γ N2 crystals using an intermolecular potential from ab initio calculations. This potential contains electrostatic (multipole) interactions, up to all R−9 terms inclusive, anisotropic dispersion interactions up to all R−10 terms inclusive, and anisotropic overlap interactions caused by charge penetration and exchange between the molecules. The lattice constants, cohesion energy, the frequencies of the translational phonon modes and the Gruneisen parameters for the librational modes are in good agreement with experimental values, confirming the quality of the potential. The frequencies of the librational modes and those of the mixed modes are less well reproduced, especially at temperatures near the α–β phase transition. Probably, the self‐consistent phonon method used does not fully account for the anharmonicity in the librations.

A potential energy surface for the ground state of acetylene, H2C2([Xtilde] 1Σ g +)

Abstract: A potential energy function has been derived for the ground state surface of C2H2 as a many-body expansion. The 2- and 3-body terms have been obtained by preliminary investigation of the ground state surfaces of CH2([Xtilde] 3 B 1) and C2H([Xtilde] 2Σ+). A 4-body term has been derived which reproduces the energy, geometry and harmonic force field of C2H2. The potential has a secondary minimum corresponding to the vinylidene structure and the geometry and energy of this are in close agreement with predictions from ab initio calculations. The saddle point for the HCCH-H2CC rearrangement is predicted to lie 2·530 eV above the acetylene minimum.

Rotationally inelastic collisions of LiH with He. I. Ab initio potential energy surface

Abstract: The diagrammatic many‐body perturbation theory is applied through third order in the correlation energy to the interaction potential between He and a rigid LiH molecule. The ab initio calculations are used to derive an analytic representation of the potential surface in terms of orthogonal polynomials. Several different basis sets are employed to demonstrate the sensitivity of the energies to the computational techniques. The resulting potential surfaces are highly anisotropic with respect to the LiH center‐of‐mass and allow for a weak binding (∼7 meV) of the He to the Li end of the LiH axis.

Spectroscopy of the NaH, NaD, KH, and KD X 1Σ+ ground state by laser excited fluorescence in a high frequency discharge

Abstract: By laser excited fluorescence of the hydrides obtained in a discharge, the ground state vibrational levels of NaH, NaD, KH, and KD have been observed up to v″=15, 20, 14, and 16, respectively, instead of v″=8, 2, 4, and 4 by conventional spectroscopy. Experimental values of Gv″, Bv″, and Dv″ are obtained. Spectroscopic parameters and RKR potential curves are calculated. In NaH and NaD, a comparison can be made with ab initio calculations.

The effects of basis set quality and configuration mixing in ab initio calculations of the ionization potentials of the nitrogen molecule

Abstract: Systematic configuration interaction calculations of the adiabatic ionization potentials of N2(1Σg+) to the X 2Σg+, A 2Πu and B 2Σu+ states of N2+ give best results of 15.62, 16.41, and 18.93 eV compared with observed values of 15.576, 16.694, and 18.746 eV. The computed values are near the complete basis limit for a wave function containing single and double excitations from the Hartree–Fock configuration with the Langhoff–Davidson correction for unlinked cluster quadruple excitations. The critical dependence of ionization potentials on basis is demonstrated. Vertical ionization potentials at near the N2 ground state equilibrium internuclear separation are compared for a variety of basis sets using different computational methods. In particular, an important comparison is made with the EOM calculation of Herman, Yeager, Freed, and McKoy using a ’’double‐zeta+polarization’’ basis identical to one used in this study. We conclude that agreement to within ∼0.2 eV can be expected for ionization potentials bet...

Ab Initio MRD‐CI Calculations for HAB Molecules I. Isomerization Energies for HOS, HNP and HCSi and Their Positive Ions

Abstract: A series of ab initio calculations is reported which employs the MRD-CI method in a standard AO basis of double-zeta plus polarization quality for the HSO-HOS, HPN-HNP and HSiC-HCSi isomers and their corresponding positive ions. The equilibrium geometries of each of these twelve systems has been carefully optimized (in some cases in a variety of low-lying electronic states) and the structural results are found to be in substantial agreement with the qualitative predictions of the Mulliken-Walsh MO model, although some significant deviations from this general pattern are indicated for certain of the more relatively unstable isomeric species. Unusually large isomerization energies are calculated in a number of instances and a strong preference for structures with the second-row atom at the terminal position of the molecule is noted generally. The largest and smallest distinctions in the relative stabilities of the isomers studied to date are found to occur for systems with 10 and 12 valence electrons respectively (110 kcal/mole for HCS+-HSC+ and 2 kcal/mole for ‘A’ HOS+-HSO+, for example) and from a survey of such findings it is concluded that each type of molecular orbital exhibits a characteristic effect on the magnitude of these energy differences. Finally it is pointed out that in isovalent series isomerization energies tend to increase with the electronegativity of the second-row atom in HAB systems of mixed first- and second-row composition.

The structure of singlet carbene molecules

Abstract: Application of the principles of chemical bonding to the problem of the structure of singlet carbenes CX2 leads to bent structures for X = F, OH, and NH2, and linear structures for X = Li, BeH, and BH2, in agreement with the ab initio calculations recently reported by Schoeller; these structures involve double bonds, with transfer of an electron from X to C for the first set and from C to X for the second, and the bond angle is determined by the availability of orbitals, not the electronegativity difference of C and X, which, however, helps to determine the stability of the carbene.

Magnetic single-ion anisotropy and zero-field splittings of Mn2+ ions in some low-dimensional chlorides

Abstract: The zero-field splitting parameter D of the 6S5/2 ground state of Mn2+ ions with Cl- ligands is discussed. Ab initio calculations of D, based on the relativistic spin-spin and Blume-Orbach splitting mechanisms, and an electrostatic model of the crystal field, are in poor agreement with experimental measurements for Mn2+ ions in a representative group of six chloride host compounds: CdCl2, MgCl2, CsMgCl3, CsZnCl5, TMCC and (CH2NH3)2CdCl4. In contrast, the empirical superposition model gives a consistent account of the experimental data for six-fold coordinated Mn2+ ions in this group of compounds. The dependence of the intrinsic parameter b2 on ligand distance R is found to be b2=-0.235 (R-R0) cm-1, with R0=2.692 AA, suggesting that the zero-field splitting involves a cancellation effect between two competing mechanisms. The superposition model is used to predict zero-field splittings of D=+0.019 and +0.036 cm-1 in the low-dimensional antiferromagnets TMMC and (CH3NH3)2 MnCl4 respectively.

The structure of carbon trioxide

Abstract: Open (1) and cyclic (2) singlet forms of CO3 were investigated by means of ab initio calculations. At the highest level of theory employed, MP2/6-31G* (which includes the effects of electron correlation), 2 was indicated to be much more stable than 1 and thermodynamically stable toward dissociation into CO2 and O(3P). The open form 1 has a long OO bond and can be regarded as a weak dative complex between CO2 and a singlet oxygen atom.

Dynamical calculation of satellite intensities

Abstract: A formalism to calculate molecular electronic spectra is developed which takes account of electronic configuration interaction as well as of vibronic coupling between the electronic states. The formalism allows the dynamical calculation of electronic spectra, i.e., the inclusion of the effects of the nuclear kinetic energy operator on the electronic motion. When neglecting the dynamical effects, the usual statical adiabatic and Franck‐Condon approximations are obtained as special cases. The formalism is applied to cyanogen the photoelectron spectrum of which exhibits an unexplained peculiar satellite structure. Ab initio calculations are performed which show that vibronic interaction between close lying 2Σg and 2Πu states takes place through the bending vibration. The vibronic coupling effects are enhanced by the simultaneous excitation of the totally symmetric C–N stretching vibration. Guided by these results a dynamical calculation is performed which reproduces nicely the experimental spectrum. The adia...

MCSCF potential energy surface for the high barrier adiabatic 1 4Σ− pathway of the O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction

Abstract: The collinear 4Σ− pathway for the state‐specific O+(4S)+N2(X 1Σg+) →NO+(X 1Σ+)+N(4S) reaction has been surveyed with ab initio calculations. A ninety‐nine configuration, fifteen orbital multiconfiguration self‐consistent field (MCSCF) wave function, involving the use of a double‐zeta plus polarization one‐electron basis, was developed for the long range 4Σ− state. This long range 4Σ− state has the character of O++N2 for long RNO, or of N+NO+ for long RNN, and is for most geometries, the lowest, or 1 4Σ−, state. The ab initio exothermicity computed with the present wave function is 0.93 eV, compared to an accurate experimental value of 1.10 eV. The saddle‐point in the energy surface is 8.0 eV above O++N2, with critical values of R*NN=1.48±0.02 A and R*NO =1.38±0.02 A. These values are 0.38 and 0.32 A greater than the equilibrium bond lengths of N2(X 1Σg+) and NO+(X 1Σ+). The present wave function reproduces the experimental bond lengths of these two diatomics to within 0.01 A when the third atom is removed...

Ab Initio Calculation of One‐Center Integrals of Semiempirical Theories of Valence

Abstract: In order to highlight some of the apparent contradictions between semiempirical and ab initio theories of valence, a brief review of some facets of the two approaches is presented. The recently developed effective valence shell Hamiltonian, , is then briefly introduced to bridge the gap. Results of purely ab initio Hv calculations for atoms and simple molecules are mentioned. The ab initio one-center integrals are quantitatively much different from those of semiempirical theories. However, the exact contains extra terms, three- (or higher) electron interactions, which are totally absent in semiempirical theories. This paper shows that the nonclassical, three-electron terms are incorporated into semiempirical one- and two-electron integrals, and a parametrization scheme illustrating this is presented. As an example, the carbon system is chosen and the one-center carbon semiempirical integrals are calculated from the results of ab initio calculations for C and CH. This enables discussions of questions of the degree of transferability of semiempirical one-center integrals. The one-center one- and two-electron integrals obtained from ab initio calculations by including the effects of three-electron terms are found to be close to the traditional semiempirical values.