Showing papers on "Ab initio quantum chemistry methods published in 1995"

AB initio molecular dynamics for liquid metals

Abstract: In recent years, ab initio molecular dynamics (MD) techniques have made a profound impact on the investigation of the structure of the electronic and dynamic properties of liquid and amorphous materials In this paper, recent developments in this field are reviewed and it is shown that the exact calculation of the electronic groundstate at each MD timestep is feasible using modern iterative matrix diagonalization algorithms Together with the use of ultrasoft pseudopotentials, ab initio MD simulations can be extended to open-shell transition metals with a high density of states at the Fermi-level The technique is applied to a number of interesting cases: (a) liquid simple metals (Li, Na, Al, Ge), (b) liquid transition metals (Cu, V), and (c) the transition from a liquid metal to an amorphous semiconductor by the rapid quenching of Ge

IMOMM: A new integrated ab initio + molecular mechanics geometry optimization scheme of equilibrium structures and transition states

Abstract: A new computational scheme integrating ab initio and molecular mechanics descriptions in different parts of the same molecule is presented. In contrast with previous approaches, this method is especially designed to allow the introduction of molecular mechanics corrections in full geometry optimizations concerning problems usually studied through ab initio calculations on model systems. The scheme proposed in this article intends to solve some of the systematic error associated with modeling through the use of molecular mechanics corrections. This method, which does not require any new parameter, evaluates explicitly the energy derivatives with respect to geometrical parameters and therefore has a straightforward application to geometry optimization. Examples of its performance on two simple cases are provided: the equilibrium geometry of cyclopropene and the energy barriers on SN2 reactions of alkyl chloride systems. Results are in satisfactory agreement with those of full ab initio calculations in both cases. © 1995 by John Wiley & Sons, Inc.

Ab initio molecular dynamics simulation of the solvation and transport of hydronium and hydroxyl ions in water

Abstract: Charge defects in water created by excess or missing protons appear in the form of solvated hydronium H3O+ and hydroxyl OH− ions. Using the method of ab initio molecular dynamics, we have investigated the structure and proton transfer dynamics of the solvation complexes, which embed the ions in the network of hydrogen bonds in the liquid. In our ab initio molecular dynamics approach, the interatomic forces are calculated each time step from the instantaneous electronic structure using density functional methods. All hydrogen atoms, including the excess proton, are treated as classical particles with the mass of a deuterium atom. For the H3O+ ion we find a dynamic solvation complex, which continuously fluctuates between a (H5O2)+ and a (H9O4)+ structure as a result of proton transfer. The OH− has a predominantly planar fourfold coordination forming a (H9O5)− complex. Occasionally this complex is transformed in a more open tetrahedral (H7O4)− structure. Proton transfer is observed only for the more waterlike (H7O4)− complex. Transport of the charge defects is a concerted dynamical process coupling proton transfer along hydrogen bonds and reorganization of the local environment. The simulation results strongly support the structural diffusion mechanism for charge transport. In this model, the entire structure—and not the constituent particles—of the charged complex migrates through the hydrogen bond network. For H3O+, we propose that transport of the excess proton is driven by coordination fluctuations in the first solvation shell (i.e., second solvation shell dynamics). The rate‐limiting step for OH− diffusion is the formation of the (H7O4)− structure, which is the solvation state showing proton transfer activity.

Ab initio calculations and force field development for computer simulation of polysilanes

Abstract: An all-atom force field for computer simulation of polysilanes, including their alkyl and phenyl side-chain derivatives, is developed based on ab initio calculations and liquid-state simulations of simple silane molecules. Validation of the force field shows good agreement with experimental data of isolated and condensed silanes and polysilanes. Conformational structures and energies of polysilane and poly(dimethylsilane) were studied based on ab initio calculations and the force field developed. It is found that the all-trans backbone of isolated polysilane is more stable than the gauche. Poly(dimethylsilane) has a helical backbone in the gas phase but is all-trans in the crystal due to the interchain nonbonded interaction

The anharmonic force field of ethylene, C2H4, by means of accurate ab initio calculations

Abstract: The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the C-12 isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals could be reproduced to better than 10 per centimeter, except for three cases of severe Fermi type 1 resonance. The problem with these three bands is identified as a systematic overestimate of the Kiij Fermi resonance constants by a factor of two or more; if this is corrected for, the predicted fundamentals come into excellent agreement with experiment. No such systematic overestimate is seen for Fermi type 2 resonances. Our computed harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch-bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the v8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper.

Ab initio study of the nonlinear optical properties of urea: Electron correlation and dispersion effects

Abstract: We present the results of ab initio calculations on the first-, second-, and third-order molecular polarizabilities of urea. An efficacious general finite field perturbation approach, previously applied in the case of paranitroaniline, is extended to the evaluation of axial and nonaxial components of the nonlinear responses. The validity of our numerical procedure is examined at the Hartree–Fock level of theory by comparison with analytical derivative results. The impact of electron correlation is analyzed, by calculating the optical nonlinearities at the Moller-Plesset perturbation theory level. The second-order Moller-Plesset electron correlation correction is shown: (i) to enhance the third-order polarizability y by almost a factor of 2 and (ii) to include the major correlation effects as consideration of the fourth-order correction further improves the y components by less than 20%. We also discuss the frequency-dependence of the nonlinear optical properties of urea by presenting calculations of the dynamic components at the noncorrelated level of theory for different optical processes. © 1995 John Wiley & Sons, Inc.

Class IV charge models: A new semiempirical approach in quantum chemistry

Abstract: We propose a new criterion for defining partial charges on atoms in molecules, namely that physical observables calculated from those partial charges should be as accurate as possible. We also propose a method to obtain such charges based on a mapping from approximate electronic wave functions. The method is illustrated by parameterizing two new charge models called AM1-CM1A and PM3-CM1P, based on experimental dipole moments and, respectively, on AM1 and PM3 semiempirical electronic wave functions. These charge models yield rms errors of 0.30 and 0.26 D, respectively, in the dipole moments of a set of 195 neutral molecules consisting of 103 molecules containing H, C, N and O, covering variations of multiple common organic functional groups, 68 fluorides, chlorides, bromides and iodides, 15 compounds containing H, C, Si or S, and 9 compounds containing C-S-O or C-N-O linkages. In addition, partial charges computed with this method agree extremely well with high-level ab initio calculations for both neutral compounds and ions. The CM1 charge models provide a more accurate point charge representation of the dipole moment than provided by most previously available partial charges, and they are far less expensive to compute.

Ab Initio Calculations for Helium: A Standard for Transport Property Measurements

Abstract: For helium, the accuracy of calculated transport properties and virial coefficients based on an accurate ab initio potential now exceeds that of the best measurements. The ab initio results should be used to calibrate measuring apparatus.

Neutral rare-gas containing charge-transfer molecules in solid matrices. I. HXeCl, HXeBr, HXeI, and HKrCl in Kr and Xe

Abstract: Ultraviolet‐irradiation of hydrogen halide containing rare gas matrices yields the formation of linear centrosymmetric cations of type (XHX)+, (X=Ar, Kr, Xe). Annealing of the irradiated doped solids produces, along with thermoluminescence, extremely strong absorptions in the 1700–1000 cm−1 region. Based on isotopic substitution and halogen dependence of these bands, the presence of hydrogen and halogen atom(s) in these species is evident. In the present paper we show the participation of rare gas atom(s) in these new compounds. The evidence is based on studies of the thermally generated species in mixed rare gas matrices. The new species are assigned as neutral charge‐transfer molecules HX+Y− (Y=halogen), and their vibrational spectra are discussed and compared with those calculated with ab initio methods. This is the first time hydrogen and a rare gas atom has been found to make a chemical bond in a neutral stable compound. The highest level ab initio calculations on the existence of compounds of type HXY corroborate the experimental observations. The mechanism responsible for the formation of these species is also discussed.

Dimer reconstruction of diamond, Si, and Ge (001) surfaces.

Abstract: Ab initio calculations of structural and electronic properties of the C(001)-(2 \ifmmode\times\else\texttimes\fi{} 1) diamond surface are reported and discussed in direct comparison with Si(001) and Ge(001). Our results strongly favor a symmetric dimer reconstruction of C(001)-(2 \ifmmode\times\else\texttimes\fi{} 1) as opposed to an asymmetric dimer reconstruction of Si and Ge (001). The physical origin and quantitative nature of the dimer reconstructions are investigated systematically, and it is shown by analyzing chemical trends why Si(001) is the most subtle case for an unequivocal surface structure determination.

An Accurate ab initio Quartic Force Field and Vibrational Frequencies for CH4 and Isotopomers

Abstract: A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within +/- 6 cm(exp -1). It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 r(e), by 0.0015 A. Our best estimate for r, is 1.0862 +/- 0.0005 A.

Ab initio calculations of the quasiparticle and absorption spectra of clusters: The sodium tetramer.

Abstract: We report the first ab initio quasiparticle calculation in a real cluster ${\mathrm{Na}}_{4}$ within Hedin's $\mathrm{GW}$ approximation for the valence electron self-energy. Our approach avoids the summations over empty states, and also eliminates the problem of residual interactions between the periodic images. Self-energy corrections open the local density approximation gap by more than 2 eV; finite-size effects on screening are shown to play an important role. The absorption spectrum calculated by including excitonic effects using our ab initio screened interaction gives a good account of the experimental photodepletion data.

Electrostatic decoupling of periodic images of plane‐wave‐expanded densities and derived atomic point charges

Abstract: The density of a molecule expanded in plane waves is decoupled from its periodic images using a fit to atom‐centered Gaussians, which reproduces the long‐range electrostatic potential of the original density. The interaction energy between the cluster and its periodic images is calculated by an Ewald summation. The method has been applied to self‐consistent ab initio molecular dynamics calculations of charged and polar molecules. An atomic point charge model of the charge density is obtained, which can be used for classical molecular dynamics models and to couple classical and quantum mechanical simulations.

An extended basis set ab initio study of alkali metal cation–water clusters

Abstract: Ionic clusters comprised of a single alkali metal cation and up to eight water molecules were studied at the Hartree–Fock and correlated levels of theory using the correlation consistent sequence of basis sets. Estimates of the degree of convergence in the computed properties with respect to the complete basis set limit were facilitated by the underlying systematic manner in which the correlation consistent sets approach completeness. In favorable cases, improved property values could be obtained by fitting finite basis set results with a simple analytical expression in order to extrapolate to the complete basis set limit. The sensitivity of structures and binding energies were analyzed with regard to the inclusion of valence and core‐valence correlation recovery at the MP2, MP4, and CCSD(T) levels of theory. The replacement of metal core electrons and the introduction of relativistic contributions via effective core potentials was compared to corresponding all‐electron results.

A study of some organic reactions using density functional theory

Abstract: Twelve organic reactions (six closed shell and six radical) were studied using semiempirical, traditional ab initio and density functional methodologies. Full geometry optimizations of all species, both minima and transition states, were performed, and calculated geometries and barrier heights compared with experimental data. Our results demonstrate that although currently available density functionals tend to underestimate barrier heights, especially for radical reactions—in some cases reactions with low barriers are predicted to be essentially barrier free—they provide a significant improvement over standard methods. The adiabatic connection method recently proposed by Becke [J. Chem. Phys. 98, 5648 (1993)], in which a portion of the exact Hartree–Fock exchange is mixed in to the density functional, looks very promising.

Space-time method for ab initio calculations of self-energies and dielectric response functions of solids.

Abstract: We present a new method for efficient, accurate calculations of many-body properties of periodic systems. The main features are (i) use of a real-space/imaginary-time representation, (ii) avoidance of any model form for the screened interaction W, (iii) exact separation of W and the self-energy Σ into short- and long-ranged parts, and (iv) the use of novel analytical continuation techniques in the energy domain. The computer time scales approximately linearly with system size. We give results for jellium and silicon, including the spectral function of silicon obtained from the Dyson equation.

Magnetic nanostructures:4d clusters on Ag(001).

Abstract: We perform ab initio calculations for the electronic structure of $4d$ transition-metal clusters at the (001) surface of Ag and determine the magnetic moments. Dimers, linear chains, and plane islands are investigated, all showing a strong tendency for magnetism. We also compare our results with calculations for free clusters. Because of the hybridization with the substrate and with the adatoms in the clusters, the maximum of the moment curve is shifted to large valences. For all investigated structures Ru and Rh clusters are magnetic.

Neutral rare‐gas containing charge‐transfer molecules in solid matrices. II. HXeH, HXeD, and DXeD in Xe

Abstract: Photolysis of hydrogen halides (and some other hydrogen containing small molecules) in solid Xe yields in a two step process charged centers, one of them being XeHXe+. Annealing of the irradiated doped solids produces, in addition to H–Xe–Y (Y=Cl, Br, or I) species characterized by us previously, a fairly strong doublet at 1181 and 1166 cm−1 and a weak absorption at 701 cm−1. Deuterated precursors yield a doublet at 846 and 856 cm−1. Also peaks belonging to mixed H/D form are found, indicating that the absorbing species contains two H/D atoms. The new species responsible for these absorptions are assigned as neutral linear centrosymmetric HXeH, HXeD, and DXeD. The nature of the bonding can be understood in terms of the resonance between the two ionic forms HXe+H− and H−Xe+H, analogously to the valence bond description of the well known XeF2. The pseudopotential (LANL1DZ) ab initio calculations at the MP2 level are in good agreement with the observed spectra.

Structural quantum effects and three-centre two-electron bonding in CH + 5

Abstract: HYPERCOORDINATE carbonium ions can be formed by protonating saturated hydrocarbons with superacids1–3. As this leaves a deficiency of bonding electrons, the resulting non-classical carbocations contain bonds in which two electrons are shared between three nuclei. Protonated methane, CH+5, might be seen as the prototype of such species1–3. But recent calculations4,5 have suggested that all five C–H bonds are effectively equivalent and exchange dynamically very rapidly. It was therefore concluded4 that CH+5 is a highly fluxional molecule without a definite structure, in which the representation in terms of three-centre two-electron bonding is misleading. Here we use a recently developed technique6 to perform ab initio electronic structure calculations that include quantum effects of the nuclei. We find that, although there are prominent quantum-mechanical effects on the structure, including fluxional-ity, pseudo-rotations and hydrogen scrambling, the quantum ground state is nevertheless dominated on average by configurations in which an H2 moiety is attached to a CH3 group forming a three-centre two-electron bond. To this extent, CH+5 should therefore resemble other carbonium ions.

Ab initio electronic-structure calculations for II-VI semiconductors using self-interaction-corrected pseudopotentials.

Abstract: We report results of an efficient approach for performing self-consistent ab initio calculations of structural and electronic properties of II-VI semiconductors which overcomes to a large extent well-known shortcomings of the local-density approximation (LDA) for these d-band compounds. Dominant atomic self-interaction corrections are taken into account by employing appropriately constructed pseudopotentials in the framework of standard LDA calculations. Our results for ZnO, ZnS, CdS, and CdSe are in excellent agreement with a whole body of experimental data.

Thermal and vibrational‐state selected rates of the CH4+Cl↔HCl+CH3 reaction

Abstract: We present direct ab initio dynamics studies of thermal and vibrational‐state selected rates of the hydrogen abstraction CH4 +Cl↔CH3+HCl reaction. Rate constants were calculated within the canonical variational transition state theory formalism augmented by multidimensional semiclassical tunneling corrections. A vibrational diabatic model was used for vibrational‐state selected rate calculations, particularly for exciting the CH4 symmetric stretching and umbrella bending modes. The potential energy information was calculated by a combined density functional and molecular orbital approach. Becke’s half‐and‐half (BH&H) nonlocal exchange and Lee–Yang–Parr (LYP) nonlocal correlation functionals (BH&HLYP) were used with the 6‐311G(d,p) basis set for determining structures and frequencies at the stationary points and along the minimum energy path (MEP). Energetics information was further improved by a series of single point spin‐projected fourth‐order Mo/ller–Plesset perturbation theory (PMP4(SDTQ)) calculation...

Structures, energetics, and spectra of aqua-sodium(i) : thermodynamic effects and nonadditive interactions

Abstract: Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)] Various configurations were investigated to find the stable structures of the clusters The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer‐shell water molecules The thermodynamic quantities of the stable structures were compared with experimental data available Entropy‐driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution The nonadditive interactions in the clusters are found to be

The performance of density‐functional/Hartree–Fock hybrid methods: Cationic transition‐metal methyl complexes MCH+3 (M=Sc–Cu,La,Hf–Au)

Abstract: Hybrid methods, including a mixture of Hartree–Fock exchange and density functional exchange‐correlation treatment have been applied to the cationic methyl complexes MCH+3 of the first and third‐row transition metals (M=Sc–Cu,La,Hf–Au). Bond dissociation energies and optimum geometries obtained with the ‘‘Becke‐Half‐and‐Half‐Lee–Yang–Parr’’ and ‘‘Becke‐3‐Lee–Yang–Parr’’ functionals and from calibration calculations employing quadratic configuration interaction with single and double excitations and with a perturbative estimate of triple excitations are reported. A comparison of the results for the 3d‐block species to earlier high‐level ab initio calculations and experimental data is carried out in order to assess the reliability of hybrid methods as a practical tool in organometallic chemistry. Furthermore, the bond dissociation energies of the cationic 5d‐block transition‐metal methyl complexes, many of which have not been investigated so far, are predicted.

Ab initio molecular dynamics of H2O adsorbed on solid MgO

Abstract: The Car–Parrinello method has been applied to study the adsorption of water on solid magnesium oxide with surface defects. A step consisting of an (100) and an (010) surface on an (011) base plane allows us to model the experimentally observed microfaceting. In and on this step dissociation of water into a hydroxyl group and a H‐atom took place following a complicated pathway only accessible by the simulation of thermal motion. Under comparable conditions physisorption only was observed on a regular (001) plane. This solves an experimental controversy and it is in agreement with the observation, that disordered surfaces are more active in initiating the dissociation of the water molecules. Our work allows us to identify an important active center. We can also account for the experimentally observed broadening and shifting to the red of the stretching mode of hydrogen bonded hydroxyl groups, and we provide a detailed explanation of the origin of this effect. This allows us to verify earlier theories of hydrogen bonding such as that of the adiabatic separation of the proton dynamics.