Showing papers on "Ab initio quantum chemistry methods published in 1985"

Stresses in semiconductors: Ab initio calculations on Si, Ge, and GaAs.

Abstract: Explicit formulas for the calculation of stress are presented based on the stress theorem and the local-density-functional approximation. Norm-conserving pseudopotentials are applied in a plane-wave basis for calculations on the semiconductors Si, Ge, and GaAs. Besides the lattice constants and bulk moduli, complete sets of elastic constants are given, together with the optical \ensuremath{\Gamma} phonon frequencies and internal-strain parameter \ensuremath{\zeta}. Electronic charge density structure factors, deformation potentials, and strain-induced splittings of phonons are given, as well as the nonlinear third-order elastic constants. Good agreement with experiment is found throughout, except for persistent deviations from the x-ray diffraction values for \ensuremath{\zeta}.

Ground- and excited-state properties of Li2 and Li2+ from ab initio calculations with effective core polarization potentials

Abstract: Potential curves, vibrational term values, and spectroscopic constants for eighteen low-lying electronic states of Li2 and eight electronic states of Li2+ are obtained from all-electron SCF/valence CI calculations including core polarization effects by an effective potential. Previous theoretical results for experimentally known states appear to be significantly improved. Deviations from experimental data amount to ΔDc ⩽ 50 cm−1, Δωc ⩽ 0.5 cm−1, ΔRc ⩽ 0.005 A, and ΔBc ⩽ 0.002 cm−1, respectively. For unobserved or uncertain data predictions are made whose accuracy is believed to be given by these bounds. In several cases, the purely theoretical data have been combined with experimental information to yield even more precise predictions, e.g. for the hump height of the B-state potential, the ground-state ionization energy and spectroscopic constants of the I3Πu state. Transition moment functions and radiative lifetimes are obtained for all molecular states under consideration. Our calculated A-state lifetimes for unperturbed rovibronic levels are within the experimental uncertainty of 1–2%.

The tetrahedral framework in glasses and melts — inferences from molecular orbital calculations and implications for structure, thermodynamics, and physical properties

Abstract: Results of ab initio molecular orbital (MO) calculations provide a basis for the interpretation of structural and thermodynamic properties of crystals, glasses, and melts containing tetrahedrally coordinated Si, Al, and B. Calculated and experimental tetrahedral atom-oxygen (TO) bond lengths are in good agreement and the observed average SiO and AlO bond lengths remain relatively constant in crystalline, glassy, and molten materials. The TOT framework geometry, which determines the major structural features, is governed largely by the local constraints of the strong TO bonds and its major features are modeled well by ab initio calculations on small clusters. Observed bond lengths for non-framework cations are not always in agreement with calculated values, and reasons for this are discussed in the text. The flexibility of SiOSi, SiOAl, and AlOAl angles is in accord with easy glass formation in silicates and aluminosilicates. The stronger constraints on tetrahedral BOB and BOSi angles, as evidenced by much deeper and steeper calculated potential energy versus angle curves, suggest much greater difficulty in substituting tetrahedral B than Al for Si. This is supported by the pattern of immiscibility in borosilicate glasses, although the occurrence of boron in trigonal coordination is an added complication. The limitations on glass formation in oxysulfide and oxynitride systems may be related to the angular requirements of SiSSi and Si(NH)Si groups. Although the SiO and AlO bonds are the strongest ones in silicates and aluminosilicates, they are perturbed by other cations. Increasing perturbation and weakening of the framework occurs with increasing ability of the other atom to compete with Si or Al for bonding to oxygen, that is, with increasing cation field strength. The perturbation of TOT groups, as evidenced by TO bond lengthening predicted by MO calculations and observed in ordered crystalline aluminosilicates, increases in the series Ca, Mg and K, Na, Li. This perturbation correlates strongly with thermochemical mixing properties of glasses in the systems SiO2-M 1 /+ AlO2 and SiO2-M n+O n/2 (M=Li, Na, K, Rb, Cs, and Mg, Ca, Sr, Ba, Pb), with tendencies toward immiscibility in these systems, and with systematics in vibrational spectra. Trends in physical properties, including viscosity at atmospheric and high pressure, can also be correlated.

Interaction of magnetic impurities in Cu and Ag

Abstract: Ab initio calculations for the electronic structure of pairs of magnetic impurities like Cr, Mn and Fe in Cu and Ag are presented. The calculations are based on density functional theory and apply the KKR Green function method. The authors calculate the local densities of states and the local moments for both the ferromagnetic and antiferromagnetic pair configurations. In particular, they estimate the energy difference between these configurations for different interatomic impurity distances. They show that this RKKY-type energy can be obtained in first order from the difference between the single-particle energies of these configurations calculated with the same 'frozen' impurity potential. In both hosts, two Fe impurities couple strongly ferromagnetically on nearest-neighbour sites, whereas Cr impurities show a strong antiferromagnetic interaction. Mn impurities are an intermediate case, interacting weakly antiferromagnetically.

Computer simulations of aqueous electrolyte solutions

Abstract: This review concentrates mainly on the structural and dynamical properties of aqueous electrolyte solutions derived from MD simulations with the ST2 and an improved Central Force model for water. The ion-water pair potentials are either calculated by modelling the ions as Lennard-Jones spheres with an elementary charge at the center or based on ab initio calculations. The concentrations ranged from 0.55 to 2.2 molal with 200 water molecules in the basic periodic cube. The simulations extended over 10 ps. The structural properties of the solutions are discussed on the basis of radial distribution functions, the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. The dynamical properties are calculated from various autocorrelation functions. Results are presented for the influence of the ions on self-diffusion coefficients, hindered translations, librations and internal vibrations of the water molecules.

Deuterium quadrupole coupling constants. A theoretical investigation

Abstract: Deuterium quadrupole coupling constants were obtained for 39 sites from ab initio SCF calculations. An accuracy comparable to the experimental one is reached with a moderately large basis set of very high local quality. Comparison with experiment shows that electron correlation does not contribute substantially to this property. The experimental values for DNCO and DFCO should be reexamined, the value for CD3Cl in the literature was not properly transformed to the bond axes system, and the experimental value for ND3 agrees with the calculated one, thus showing that a dynamical model is not necessary. Several empirical relations are presented, and a simple classical model is proposed which relates the deuterium quadrupole coupling constant to the bond length with high accuracy. A theoretical study of the HCOOH dimer gives some insight into the importance of different mechanisms which lower the quadrupole coupling constant in the solid.

Structure of ice Ih. Ab initio two- and three-body water-water potentials and geometry optimization

Abstract: The OO distance in ice (2.76 A) is much shorter than in water dimer (2.98 A). No first principle potential function has successfully described the observed OO shrinkage. We have calculated water–water two‐body interaction potentials with an ab initio MO method by varying not only the OO distance but also the OH distance. New analytical fits of two‐body potential functions have been obtained. The nearest‐neighbor three‐body potential has been evaluated for proton‐ordered ice–Ih structures. With ab initio one, two, and nearest‐neighbor three‐body potentials, ∠HOH fixed at the monomer value, we have been able to obtain ROO=2.79 A, ROH=0.977 A with the binding energy of 15.8 kcal/mol per H2O molecule for proton‐ordered antiferroelectric ice Ih and ROO=2.85 A, ROH=0.972 A with the binding energy of 14.3 kcal/mol per molecule for ferroelectric ice Ih. The three‐body interaction, aided by the two‐body interaction, contributes to the OO shrinkage. Factors that would favor larger ROH stretch and ROO shrinkage have...

The EF, GK, and HH̄ 1Σg+ states of hydrogen. Improved ab initio calculation of vibrational states in the adiabatic approximation

Abstract: Electronic energies and diagonal energy corrections for nuclear motion are calculated for the first three excited 1Σg+ states of H2 at some 70 internuclear separations in the interval R(a.u.) ∈{1,15}. More accurate electronic wave functions comprising, respectively, 129, 118, and 110 terms for the EF, GK, and HH states, and a new method for the evaluation of the relevant integrals are used. The maximum values of the adiabatic energy corrections are found to be larger than the previously published values by 60 cm−1 (EF at R=3.2 a.u.), 55 cm−1 (GK at R=2.85 a.u.), and 38 cm−1 (HH at R=2.95 a.u.), while the electronic energies at the same values of R are now lower by −6, −26, and −7 cm−1. The adiabatic ab initio vibrational energies of the EF, v=0, levels lie 1.9 (H2) and 1.4 cm−1 (HD and D2) above the experimental values. All higher vibrational levels of the three electronic states are appreciably affected by nonadiabatic energy shifts.

The distortion of the ring in monosubstituted benzene derivatives: a molecular orbital study

Abstract: Ab initio calculations using the 6-31G basis set have been carried out on benzene and the monosubstituted derivatives with CH3, NH2, OH, F, NO, CHCH2, CCH, CCF COO− O−, and with NO2, CHO, CHNH, COOH, CFO, CN, NC and NH3+, as substituent groups. The C- and H-atoms of the ring and the substituent group atom directly attached to it were assumed to lie in the same plane, and a particular orientation was assumed for certain groups, otherwise full geometry optimization was employed. Trends in the following parameters are discussed — the ipso angle, the lengths of the CC bonds which include the ipso angle, the CC and CH bond lengths, nonbonded C ⋯ C and H ⋯ H distances, the ring area, and the tilt of the unsymmetrical substituent groups with respect to the ring axis. Additional calculations at the 6–31G* level on benzene and the F, CN and NH2 derivatives show the trends to be unaffected by the inclusion of polarization functions on the heavy atoms. In selected cases the calculated geometries are compared with microwave and X-ray diffraction results. Comparison is also made with the Mulliken population analysis of Hehre, Radom and Pople (1972) who used the STO-3G basis set and standard geometry. The difference in energy between that for the optimized structure and that for a reference structure with optimized benzene ring geometry and the optimized geometry for the attachment and substituent group has been calculated for the F, NO2, OH, CN, CHCH2 and O− derivatives. The small values, less than 1 kcal mol−1, except for O−, suggest that the actual physical state might well be a mixture of structures having slightly different ring geometries.

The N2-N2 interaction

Abstract: Results are reported from large ab initio calculations at the CPF level for the N2 dimer. An analytical potential function was derived from the ab initio data points and used in the molecular dynamics simulation of liquid N 2. The results for the N2 dimer and the liquid state properties are compared with experimental work and other theoretical results.

Infrared intensities of amide modes in N‐methylacetamide and poly(glycine I) from ab initio calculations of dipole moment derivatives of N‐methylacetamide

Abstract: The infrared intensities of the amide modes in N‐methylacetamide (NMA) and poly(glycine I) (PGI) have been studied using ab initio dipole moment derivatives obtained for the peptide group in NMA and an empirical force field refined for PGI. Good agreement is found between the calculated transition moment magnitudes and directions of the amide I and II modes and experimental intensity and dichroism data. By analyzing the separate contributions of each internal coordinate to the total intensity, we are able to understand in detail the origins of the IR intensities of the amide modes. Besides demonstrating one approach by which IR intensities can be studied in complex molecules and polymers, our results also provide a basis for using IR intensities in structural studies of peptides and polypeptides.

Wave functions derived by quantum modeling of the electron density from coherent x-ray diffraction: Beryllium metal.

Abstract: A single-determinant experimental Be hybrid-atom wave function is obtained by the quantum formalism of Clinton and Massa, from Larsen and Hansen's single-crystal x-ray diffraction data. Physical properties calculable from this wave function are unobtainable from current methods of diffraction analysis. The least-squares fit using all experimental data gives an $R$ factor of 0.0018, and quantitatively describes charge redistribution due to crystal bonding in agreement with ab initio calculations.

A water dimer potential based on ab initio calculations using Morokuma component analyses

Abstract: We present a new water dimer potential. It is based on carrying out Morokuma component analyses at the ab initio level on 229 water dimer geometries and fitting the energy components to analytical functional forms. To these are then added a dispersion contribution, based on the analytical dispersion energy expression of Douketis et al. (Ref. 1). This potential gives a satisfactory description of the water dimer, with minimum R(O ⋅ ⋅ ⋅ O)=2.96 A, ΔE=−5.2 kcal/mol, and minimum energy θ angle between H‐bond donor and acceptor axis of 55°, in good agreement with experimental values of 2.98±0.04 A, −5.4±0.2 kcal/mol, and 60±10°, respectively. The second virial coefficient B is also calculated in reasonable agreement with experiment, e.g., at 423 K, B (calculated) =−352, compared to B (experimental)=−332 cc/mol.

Hexasilabenzene (Si6H6). An ab initio theoretical study of its aromaticity and relative stability

Abstract: Ab initio calculations show that hexasilabenzene has approximately half the aromatic stabilization energy of benzene, and lies very close in stability to its valence Si6H6 isomers, this being in sharp contrast with the fact that benzene is by far the most stable C6H6 isomer.

Studies of silicon-phosphorus bonding

Abstract: Ab initio calculations are presented for several species containing a silicon-phosphorus bond. The types of bonding studied include "normal" single, double, and triple bonds, as well as an ylide-like structure. The latter is found to be much less strongly bound than the carbon analogue, with a smaller stretching force constant than that in silylphosphine. The insertions of silylene into the phosphine bond and of phosphinosilylene into H2 are discussed, with the former being illustrated using localized molecular orbitals along the intrinsic reaction coordinate (IRC). Silylene to silene isomerizations in both the closed-shell singlet and the lowest triplet states of SiPH3 are analyzed in the same manner.

Ab initio multireference CI determinations of the electron affinity of carbon and oxygen

Abstract: In order to better understand the requirements for obtaining ab initio electron affinities which agree with experiment to within 0.1–0.2 eV, a systematic study has been made of the effects of variations in basis set and choice of CI reference space configurations. Using balanced [8s, 5p, 4d, 2f, 1g] contracted Gaussian basis sets it was possible to obtain values of 1.22 eV for carbon (experimental=1.27 eV) and 1.29 eV for oxygen (experimental=1.46 eV). Enlargement of the (s,p) portion of the basis to near Hartree–Fock limit quality resulted in substantially more correlation energy being recovered but no significant change in the computed electron affinities.

Vibrational spectra of epoxypropane

Abstract: Vibrational band assignments for epoxypropane were derived from experimental vibrational spectral data and ab initio calculations with a 6‐31G basis. Using the ab initio normal coordinates and bond moment theory, vibrational circular dichroism (VCD) parameters for (S)‐epoxypropane were calculated and compared to experimental VCD observations.

Local geometry maps and conformational transitions between low-energy conformers of N-acetyl-N′-methyl glycine amide: An ab initio study at the 4–21g level with gradient relaxed geometries

Abstract: Energy pathways between the α R , β′, C 7 eq and β-regions of the conformational energy surface of N -acetyl N ′-methyl glycine amide were obtained by SCF ab initio calculations on the 4–21G level with gradient geometry optimization at each point. The calculations point to the possibility that no barrier exists at this computational level between α R and β′. The variation of geometry (bond distances and bond angles) with conformation is analyzed in detail and the geometrical parameters which should be treated as variables in both empirical energy calculations and, possibly, in the fitting of polypeptide chains in proteins by X-ray methods, are identified. The study shows that, in general, Local Geometry Maps (of conformationally dependent structural trends) are as important as Local Energy Maps for the characterization of peptide systems.

On the use of the Breit–Pauli approximation for evaluating line strengths for spin‐forbidden transitions: Application to NF

Abstract: An ab initio treatment of the Breit–Pauli spin‐orbit interaction is presented for use with large (state‐averaged) MCSCF/CI wave functions and Cartesian Gaussian atomic orbitals. The first order perturbation theory equation which results from treating the Breit–Pauli interaction (Hso) as a perturbation to the nonrelativistic Born–Oppenheimer Hamiltonian (H0) is formulated in the configuration state function basis thereby permitting the coupling of all roots of H0 into the perturbed wave function. The basic atomic orbital integrals of Hso over Cartesian Gaussian functions are evaluated using a Rys quadrature approach. The method presented here is used to consider the spin forbidden transitions b 1Σ+→X 3Σ− and a 1Δ→X 3Σ− in NF. A comparison with a frequently used approach based on the few lowest eigenfunctions of H0 is presented.