Showing papers on "Ab initio published in 1994"

Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

Abstract: We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Interatomic Potentials from First-Principles Calculations: The Force-Matching Method

Abstract: We present a new scheme to extract numerically optimal interatomic potentials from large amounts of data produced by first-principles calculations. The method is based on fitting the potential to ab initio atomic forces of many atomic configurations, including surfaces, clusters, liquids and crystals at finite temperature. The extensive data set overcomes the difficulties encountered by traditional fitting approaches when using rich and complex analytic forms, allowing to construct potentials with a degree of accuracy comparable to that obtained by ab initio methods. A glue potential for aluminium obtained with this method is presented and discussed.

Accurate First Principles Calculation of Molecular Charge Distributions and Solvation Energies from Ab Initio Quantum Mechanics and Continuum Dielectric Theory

Abstract: In this paper, we combine high-level ab initio quantum chemical calculations with a continuum description of the solvent to obtain accurate solvation free energies of organic solutes in water. By using correlated wave functions at the generalized valence bondperfect pairing (GVB-PP) level, we are able to efficiently produce accurate gas-phase charge distributions. These are then used to obtain solvation energies in a self-consistent formalism which cycles through quantum chemical calculations in the solvent reaction field and continuum electrostatic calculations utilizing polarized solute charges. An average error of 0.6 kcal/mol for solvation energies is obtained for 29 molecules. A systematic discrepancy between theory and experiment is obtained for the difference in solvation free energy between several methylated and unmethylated primary amines and amides. This poses a major puzzle in theoretical modeling of solvation effects.

An ab Initio CFF93 All-Atom Force Field for Polycarbonates

Abstract: An all-atom CFF93 force field for polycarbonates based on ab initio calculations is reported. Force field parameters are derived by fitting to quantum mechanical total energies, first and second derivatives of total energies, and electrostatic potentials, all generated from ab initio quantum mechanical calculations on model compounds at HF/6-31GS level of theory. Valence parameters and ab initiocharges are then scaled to correct for differences between experiment and the Hartree-Fock approximation. The van der Waals parameters and the scaling factors for atomic partial charges are determined from crystal structures. Based on the force field, molecular mechanics calculations are performed for several model compounds, and the results are compared with experimental values and with the results of the ab initio calculations.

Derivation of class II force fields. I: methodology and quantum force field for the alkyl functional group and alkane molecules

Abstract: A new method for deriving force fields for molecular simulations has been developed. It is based on the derivation and parameterization of analytic representations of the ab initio potential energy surfaces. The general method is presented here and used to derive a quantum mechanical force field (QMFF) for alkanes. It is based on sampling the energy surfaces of 16 representative alkane species. For hydrocarbons, this force field contains 66 force constants and reference values. These were fit to 128,376 quantum mechanical energies and energy derivatives describing the energy surface. The detailed form of the analytic force field expression and the values of all resulting parameters are given. A series of computations is then performed to test the ability of this force field to reproduce the features of the ab initio energy surface in terms of energies as well as the first and second derivatives of the energies with respect to molecular deformations. The fit is shown to be good, with rms energy deviations of less than 7% for all molecules. Also, although only two atom types are employed, the force field accounts for the properties of both highly strained species, such as cyclopropane and methylcyclopropanes, as well as unstrained systems. The information contained in the quantum energy surface indicates that it is significantly anharmonic and that important intramolecular coupling interactions exist between internals. The representation of the nature of these interactions, not present in diagonal, quadratic force fields (Class I force fields), is shown to be important in accounting accurately for molecular energy surfaces. The Class I1 force field derived from the quantum energy surface is characterized by accounting for these important intramolecular forces. The importance of each

First-principles calculations of the energetics of stoichiometric TiO2 surfaces.

Abstract: We present self-consistent ab initio total-energy calculations of the equilibrium relaxed structures and surface energies of the stoichiometric (1\ifmmode\times\else\texttimes\fi{}1) (110), (100), (001), and (011) surfaces of ${\mathrm{TiO}}_{2}$. The relaxations of atoms on these surfaces are found to be substantial, and are responsible for a large reduction of the calculated surface energies. A Wulff construction is used to display the relative energetics of these surfaces. The (100) surface is found to be stable with respect to forming macroscpic (110) facets, while the (001) surface is nearly unstable with respect to forming macroscopic (1\ifmmode\times\else\texttimes\fi{}1) (011) facets. These results shed light on published experimental results on the structures of these surfaces.

Exact Kohn-Sham scheme based on perturbation theory

Abstract: An exact formal Kohn-Sham scheme is derived with the help of perturbation theory. Through the introduction of a basis set this Kohn-Sham scheme can be used to perform, in principle, exact Kohn-Sham calculations. As a demonstration, only zeroth- and first-order terms in the underlying perturbation theory are considered. As a result an exact basis set ``exchange-only'' method is obtained. The presented perturbation theory expansions of the exchange-correlation energy and potential may serve as a starting point for the development of new approximate exchange-correlation functionals based on Kohn-Sham orbitals and eigenvalues and may be used to check conventional exchange-correlation functionals. The formal structures of the ab initio and the introduced density-functional treatment of electronic systems are compared.

Molecular potential energy surfaces by interpolation

Abstract: A moving interpolation technique which provides an accurate representation of potential energy surfaces for polyatomic molecules is presented. The method uses the ab initio energy, energy gradient, and second derivatives calculated at dynamically important configurations. The interpolant of the energy and its derivatives converges to the exact value with increasing number of data. A procedure is given for finding the optimum configurations at which ab initio calculations are performed. The method is demonstrated by application to the six‐dimensional surface of a diatomic plus diatomic reaction.

Force field for computation of conformational energies, structures, and vibrational frequencies of aromatic polyesters

Abstract: A CFF931 all-atom force field for aromatic polyesters based on ab initio calculations is reported. The force field parameters are derived by fitting to quantum mechanical data which include total energies, first and second derivatives of the total energies, and electrostatic potentials. The valence parameters and the ab initio electrostatic potential (ESP) derived charges are then scaled to correct the systematic errors originating from the truncation of the basis functions and the neglect of electron correlation in the HF/6-31G* calculations. Based on the force field, molecular mechanics calculations are performed for homologues of poly(p-hydroxybenzoic acid) (PHBA) and poly(ethylene terephthalate) (PET). The force field results are compared with available experimental data and the ab initio results. © 1994 by John Wiley & Sons, Inc.

Stability of carbon nitride solids

Abstract: An ab initio variable-cell-shape molecular-dynamics algorithm is used to investigate the stability of three carbon nitrides with composition ${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. The hexagonal \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ structure, which has been the subject of earlier investigations, is refined and its metastability is verified. Two other metastable structures are identified: a structure resembling zinc-blende CN with one C vacancy per cubic cell and a structure resembling graphitic CN with one C vacancy per four N sites. Both the refined \ensuremath{\beta} phase and the cubic phase have compressibilities comparable to that of diamond. The graphitic and the \ensuremath{\beta} phases lie very close in energy and are slightly favored over the cubic phase.

An Ab Initio Investigation of the Structure and Alkali Metal Cation Selectivity of 18-Crown-6

Abstract: We present an ab inito, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkali metal cations Li[sup +], Na[sup +], K[sup +], Rb[sup +], and Cs[sup +]. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree-Fock (RHF) level using standard basis sets (3-21G and 6-31 + G*) and relativistic effective core potentials. Electron correlation effects are determined at the MP2 level, and wave function analysis is performed by the natural bond orbital (NBO) and associated methods. The affinity of 18c6 for the alkali metal cations is quite strong (50-100 kcal mol[sup [minus]1], depending on cation type), arising largely from the electrostatic (ionic) interaction of the cation with the nucleophilic ether backbone. Charge transfer (covalent bonding) contributions are somewhat less important, only 20-50% as strong as the electrostatic interaction. Agreement of the calculated binding enthalpies and experimentally determined quantities is rather poor. For example, the binding energy for K[sup +]/18c6 (-71.5 kcal mol[sup [minus]1]) is about 30 kcal mol[sup [minus]1] stronger than that determined by experiment, and it is not clear how to reconcile this difference. Our calculations clearly show that solvation effects strongly influence cation selectivity. 48 refs., 12 figs., 5 tabs.

Ab initio calculation of the structural and electronic properties of carbon and boron nitride using ultrasoft pseudopotentials

Abstract: We present ab initio calculations of the structural, cohesive, and electronic properties of various polymorphic forms of carbon and boron nitride. Our calculations are based on ultrasoft pseudopotentials and a variational approach to the solution of the Kohn-Sham equations. Optimization of the atomic geometries is performed using total energy calculations and by minimizing the energy via a quasi-Newton quench using the Hellmann-Feynman forces. Special attention is devoted to the convergence of the results with respect to the plane-wave basis. The entire set of structural energy differences calculated in our work is in good agreement with the most accurate results obtained using a variety of different techniques---our results represent a consistent set of data based all on the same potential. We show that the use of ultrasoft potentials allows one to achieve accurate results with low cutoff energies (and hence small basis sets).

Ab initio study of piezoelectricity and spontaneous polarization in ZnO.

Abstract: We demonstrate the feasibility of ab initio studies of piezoelectricity within an all-electron scheme. The focus of our analysis is on wurtzite ZnO; for comparison, some results are also presented for the related materials BeO and ZnS. The comparative study is performed in order to understand the microscopic origin of the peculiar behavior of ZnO, whose piezoelectric response is the strongest among the tetrahedrally bonded semiconductors. In all such materials, the piezeoelectric effect results from two different terms of opposite sign: these are usually referred to as the ``clamped-ion'' and the ``internal-strain'' contributions. Cancellation among them is least effective in ZnO, where the dominant effect is due to a rigid-ion-like mechanism. Furthermore, we compute the spontaneous polarization of ZnO and we discuss the puzzling agreement between our calculated value and a very indirect experimental estimate of the same quantity.

Molecular mechanics potential for silica and zeolite catalysts based on ab initio calculations. 1. Dense and microporous silica

Abstract: A consistent force field for the simulation of aluminum-free zeolite structures is presented. The parameters are derived from results of ab initio calculations on molecular models which represent typical structural elements of zeolites: SiO 4 tetrahedra connected to chains (disilicic and trisilicic acid), rings ([SiO(OH) 2 ] n , n=3-6), and cages ([SiO 3/2 (OH)] n , n=8, 12, 24). These calculations used a «double zeta+polarization/triple zeta +polarization» basis set. The structure predicted by means of the force field obtained are compared with the results of direct ab initio calculations of the model molecules and with observed structures of dense and microporous silica. The conclusion is reached that it is possible to derive an accurate and transferable force field for molecules and solids solely based on ab initio data for molecules

Multidimensional potential energy surface for H2 dissociation over Cu(111).

Abstract: We present ab initio density functional calculations within the generalized gradient approximation for ${\mathrm{H}}_{2}$ dissociating over Cu(111). The minimum barrier for dissociation is 0.5 eV and shows large corrugation within the unit cell and a strong dependence on the molecular orientation. Dissociation is predicted to depend strongly on translational, vibrational, and rotational degrees of freedom in accordance with experiment. We show that even for a noble metal, the $d$ electrons are important for the molecule-surface interaction.

Molecular structures and normal vibrations of trifluoromethane sulfonate (CF3SO3-) and its lithium ion pairs and aggregates

Abstract: The molecular structures, harmonic vibrational frequencies, infrared absorption intensities, internal force constants, and electronic charges of the trifluoromethanesulfonate (triflate) anion, its lithium ion pairs, and several aggregates have been studied using the ab initio self-consistent field Hartree-Fock method with 3-21+G * , 6-31+G augmented with polarization functions (d type orbitals) on the sulfur atom, and 6-31+G * basis sets. The calculated frequency shifts of the υ s (SO 3 ), υ s (CF 3 ), δ s ,(CF 3 ), and υ(CS) modes and the splitting widths of the υ as (SO 3 ) modes in lithium triflate ion pairs and aggregates are compared with those observed in IR and Raman spectra of lithium triflate dissolved in some polar, aprotic solvents

A New Dimension to Quantum Chemistry: Analytic Derivative Methods in AB Initio Molecular Electronic Structure Theory

Abstract: In modern theoretical chemistry, the importance of the analytic evaluation of energy derivatives from reliable wave functions can hardly be overestimated. This monograph presents the formulation and implementation of analytical energy derivative methods in ab initio quantum chemistry. It includes a systematic presentation of the necessary algebraic formulae for all of the derivations. The coverage is limited to derivative methods for wave functions based on the variational principle, namely restricted Hartree-Fock (RHF), configuration interaction (CI) and multi-configuration self-consistent-field (MCSCF) wave functions. The monograph is intended to facilitate the work of quantum chemists, and will serve as a useful resource for graduate-level students of the field.

Ab Initio Examination of Anomeric Effects in Tetrahydropyrans, 1,3-Dioxanes, and Glucose

Abstract: Axial and equatorial structures of 2-methyl-, 2-hydroxy-, 2-methoxy-, 2-amino-, 2-fluoro-, and 2-chlorotetrahydropyran, of the 2-tetrahydropyranylammonium cation, of 2-methyl-, 2-hydroxy, and 2-methoxy-1,3-dioxane, and of the corresponding cyclohexanes have been fully optimized at the HF9/6-31G * level. NBO analysis of the Hartree-Fock wave functions confirms that the anomeric effects of hydroxy-, methoxy-, fluoro-, and chloropyrans and of glucose and methyl glucoside are indeed due to hyperconjugation. In cyclohexane, tetrahydropyran, and glucose theoretical ΔE values involving the OH and OMe substituents are nearly identical

Characterization of the S1–S2 conical intersection in pyrazine using ab initio multiconfiguration self‐consistent‐field and multireference configuration‐interaction methods

Abstract: Potential‐energy surfaces of the three lowest singlet states of pyrazine have been calculated as a function of ab initio determined ground‐state normal coordinates, using complete‐active‐space self‐consistent‐field (CASSCF) and multireference configuration interaction (MRCI) techniques. The conical intersection of the S1 and S2 adiabatic potential‐energy surfaces has been mapped out in selected subspaces spanned by the most relevant vibrational coordinates. A unitary transformation from the adiabatic to a quasidiabatic electronic representation is performed, which eliminates the rapid variations of the wave functions responsible for the singularity of the nonadiabatic coupling element. Transition‐dipole‐moment functions have been obtained in the adiabatic and in the diabatic representation. The leading coefficients of the Taylor expansion of the diabatic potential‐energy and transition‐dipole‐moment surfaces in terms of ground‐state normal coordinates at the reference geometry have been obtained at the CASSCF/MRCI level. Using a vibronic‐coupling model Hamiltonian based on this Taylor expansion, the absorption spectrum of the interacting S1–S2 manifold has been calculated, taking account of the four spectroscopically most relevant modes.

Ab initio path-integral molecular dynamics

Abstract: A path-integral molecular dynamics technique for strongly interacting atoms using ab initio potentials derived from density functional theory is implemented. This allows the efficient inclusion of nuclear quantum dispersion in ab initio simulations at finite temperatures. We present an application to the quantum cluster H 5 + .