Showing papers on "Ab initio quantum chemistry methods published in 1994"

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.

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.

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.

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).

Photoelectron spectroscopy of Cl−, Br−, and I− solvated in water clusters

Abstract: We present the photoelectron‐spectra of Cl−, Br−, and I−, solvated in water clusters‐(H2O)n, where n is 1–7, 1–16, and 1–60, respectively, taken with 7.1 eV photon energy. The vertical binding energies of the solvated anions are used to extract the solvent electrostatic stabilization energies of the anion. The photoelectron spectra of the solvated I− indicate the formation of the first solvation layer with a coordination number of six. Ab initio calculations support solvation shell closure at n=6. This conclusion is not born‐out by current molecular dynamics calculations. These calculations favor structures with a surface solvated anion (coordination number of 3–4) and reproduce (within 0.2 eV) our vertical binding energies. The fitting of the experimental binding energies of large I−(H2O)n to the models of classical electrostatic solvation is consistent with surface solvation. In the size range n=34–40 we have detected special cluster structures, with very low electrostatic stabilization.

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

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.

Vibrational Raman optical activity calculations using London atomic orbitals

Abstract: Ab initio calculations of Raman differential intensities are presented at the self-consistent field (SCF) level of theory. The electric dipole–electric dipole, electric dipole–magnetic dipole and electric dipole–electric quadrupole polarizability tensors are calculated at the frequency of the incident light, using SCF linear response theory. London atomic orbitals are employed, imposing gauge origin invariance on the calculations. Calculations have been carried out in the harmonic approximation for CFHDT and methyloxirane.

Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculations

Abstract: The P2 1 /c form of the nitronyl nitroxide 2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl oxide (NitPh) was investigated by conventional and polarized neutron diffraction. Model-free reconstruction of the spin distribution from the experimental structure factors using the recently developed 3D maximum of entropy technique gives the first direct experimental evidence for the p-shape of the spin density in the vicinity of the major spin carriers in nitroxide free radicals. Using other reconstruction methods gives individual atomic spin populations. Most of the spin density is equally shared between the four atoms of the two NO groups

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 + .

Surface core-level shifts of some 4d-metal single-crystal surfaces: Experiments and ab-initio calculations

Abstract: High resolution measurements are reported of the surface core-level shift of the 3d level for the Rh(111), Rh(110), Pd(111), Pd(110), and Ag(111) single-crystal surfaces. These measurements and earlier ones for the Mo(110), Rh(100), and Pd(100) surfaces are analyzed by ab initio calculations of the surface core-level shift. The calculations are found to reproduce well the trends of the experimental shifts with the 4d metal and with the crystal plane. The comparison between these experimental and theoretical results demonstrates the importance of proper inclusion of final-state effects for accurate calculations of surface core-level shifts. A core hole in a surface atom is found to be better screened than one in a bulk atom for the 4d metals to the left of Pd in the Periodic Table. The use of the Z+1 approximation to describe the core hole is investigated both by explicit use of this approximation and by performing calculations for 1s and 3d core holes, respectively. The Z+1 approximation is found to be well obeyed in the case of Ag whereas for the rest of the 4d transition metals it is less precise, introducing errors of typically 0.1 eV.

Bonding between co and the mgo(001) surface : a modified picture

Abstract: The interaction energy and equilibrium distance for adsorption of CO on a perfect MgO(001) surface has been calculated, using a cluster approach for representing the surface The cluster size has been varied from one single Mg2+ ion up to Mg14O5 Ab initio model potentials (AIMP) were used to embed the explicitly described cluster while the rest of the crystal was taken to be point charges The AIMP model potentials are shown to be a necessary and reliable improvement, compared with using only point charges as a model for the surrounding crystal It is found that the electrostatic attraction and Pauli repulsion almost cancel each other and the small binding energy obtained is ascribed to dispersive forces The interaction energy is calculated to be 007–009 eV (depending on which approach to basis set superposition errors corrections is taken); this is rather low in comparison with the experimental value of 03–04 eV

Time‐dependent photodissociation of methyl iodide with five active modes

Abstract: Advances in the time propagation of multidimensional wave packets are exploited to present the A‐band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time‐dependent configurations, determined by the multiconfigurational time‐dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of t...

Dynamics of Structural Rearrangements in the Water Trimer

Abstract: The internal dynamics of the hydrogen bonding network of the water trimer are investigated by tunable far-infrared laser spectroscopy. New intermolecular vibrations have been measured at 87.1 [(HzO),] and 98.1 cm-’ [(DzO),]. Symmetry restrictions produce an exact oblate symmetric rotor pattern in the spectrum, even though theory predicts the trimer structure to be an asymmetric near-planar ring. In addition, each rovibrational transition is split into a quartet. A group theoretical treatment identifies two classes of structural rearrangements to account for these effects. There is considerable current interest in the spectroscopy and dynamics of small water clusters. Experimental and theoretical investigations of these species are motivated by the quest for a detailed understanding of the intermolecular forces and dynamics of the hydrogen bonding networks that operate in the condensed phases of water and in many biological systems.’ Numerous spectroscopic? ab initio,3 and empirical studies4 have addressed the intermolecular dynamics and potential energy surface of the water dimer. A similarly detailed characterization of the water trimer will enable a quantitative comparison with the dimer that could contribute significantly to the understanding of macroscopic systems. For instance, comparison of the water dimer and trimer ab initio intermolecular potential energy surfaces (IPS) predicts that non-pairwise additive forces (“three-body” interactions that can occur in the trimer but not the dimer) contribute 10% of the total binding energy of the trimer. Other predicted “three-body” effects are shorter 0-0 distances and higher average intermolecular vibrational frequencies in the trimer than in the dimer. Similarities in internal dynamics of the dimer and trimer can suggest mechanisms for important processes, such as proton transfer, that occur in the condensed phases of water. Tunable far-infrared laser vibration-rotation-tunneling spectroscopy (FIRVRTS) has emerged as a powerful new tool for addressing such subjects.’ Pugliano and Saykally’ (PLS) recently reported the first detailed experimental study of the cyclic water trimer. In that work, an intermolecular vibration of (DzO), was measured near 89.6 cm-1. This band displayed a strongly perturbed nearsymmetric top rotational pattern with each rovibrational transition split into a quartet. Crude estimates of the 0-0 distances were made by assuming three point masses of 20 amu and adjusting their separations for optimal agreement with the reported rotational constants. The spectral splittings were interpreted as resulting from isomerization tunneling among 96 identical frameworks (48 pairs of enatiomers) via three pathways: (1) “flipping” Aktract published in Aduance ACS Abstracts, March 1, 1994. (1) Saykally, R. J.; Blake, G. A. Science 1993, 259, 1570 and references therein. (2) Fraecr,G.T.Int.Reu.Phys. Chem. 1991,10,189andreferencestberein. ( 3 ) Smith, B. J.; Swanton, D. J.; Pople, J. A,; Scbaefer, H. F., III; Radom, L. J. Chem. Phys. 1990, 92, 1240. Niesor, U.; Corongin, G.; Clementi, E.; Kneller, G.; Bbattacbaraya, D. J. Phys. Chem. 1990,94, 7949. (4) Reimen, J. R.; Watts, R. 0.; Klein, M. L. Chem. Phys. 1982,64,95 and references therein. Cieplak, P.; Kollman, P.; Lybrand, T. J. Chem. Phys. 1990,92,6755. Jorgensen, W. L.; Chandrasekhar, J.; Madura, J.; Impey, R.; Klein, M. J. Chem. Phys. 1983, 79, 926. Townsend, M.; Morse, M.; Rice, S . A. J. Chem. Phys. 1983, 79, 2496. ( 5 ) F’ugliano, N.; Saykally, R. J. Science 1992, 257, 1937. OOO2-7863/94/1516-3507$04.50/0 of a single free hydrogen from one side of the ring to the other; (2) a motion that effectively results in a Cz rotation of a single monomer about its symmetry axis; and (3) a concerted motion that reverses the sense (yclockwise” or “counterclockwise” [cwH.s.0~) of the hydrogen bonding network around the ring. That work precipitated a number of sophisticated theoretical calculations of the trimer structure, vibrational frequencies, and interconversion tunneling dynamics.611 Of these, the calculations by Fowler and SchaefeI.6 are done at the highest level. All highlevel ab initio calculations agree that the lowest energy structure is that shown in Figure 1, and that the flipping motion is nearly free; moreover, all disagree with the crudely estimated 0-0 distances. Wales’’‘’ elegant treatment of the tunneling dynamics identified three low-energy reaction paths on the twelvedimensional IPS and estimated and associated splittings within a high barrier approximation. Schiitz et al.” carried out a detailed treatment of just the three flipping coordinates, giving special attention to the implications of the very low barrier to flipping. The highest-level ab initio calculations performed to date are those of Fowler and Schaefer.6 In this paper, we report the measurement of two new intermolecular vibrations of FIRVRTS. The Berkeley tunable far infrared spectrometer systems employed in this work have been described in detail elsewhere.13 A total of 284 VRT transitions rotationally assigned to a C-type band of (H2O)o and 57 transitionsassigned toan a-type bandof (DzO)3 weremeasured. The transitions of each isotopomer were fit to a symmetric top hamiltonian and the determined molecular constants are listed in Table 1. Portions of the (H2O)s and (D2O)s data are displayed in Figure 2.

An extended basis set ab initio study of Li+(H2O)n, n=1–6

Abstract: The structures, binding energies, and enthalpies of small molecular clusters incorporating a single lithium cation and up through six waters have been determined with extended Gaussian basis sets using Hartree–Fock and post‐Hartree–Fock methods. The resulting properties are analyzed with respect to both basis set completeness and degree of correlation recovery, including core–core and core–valence effects. Although the lithium–water interaction is largely electrostatic in nature, small basis sets, lacking in polarization and near‐valence diffuse functions, drastically overestimate the strength of the bond (by 20 kcal/mol or more) and underestimate the Li+...O distance by up to 0.1 A. Their poor performance is attributable to inherent errors in describing the electric moments and polarizability of water and to large basis set superposition errors. Thus, the accuracy with which the fundamental lithium–water interaction could be modeled was primarily dependent on the quality of the Gaussian basis set and not upon the level of correlation recovery. Basis set enlargement and correlation effects both tend to reduce the strength of the Li+(H2O) bond, but produce corrections of opposite sign for the Li+...O bond length. Although correlation effects play a minor role in describing the lithium–water interaction, as the size of the cluster increases and the number of waters involved in multiple hydrogen bonds grows, correlation recovery can become significant.

Photodissociation study on Mg+(H2O)n, n=1–5: Electronic structure and photoinduced intracluster reaction

Abstract: Photodissociation spectra of Mg+(H2O)n (n=1–5) cluster ions were examined in the wavelength region from 720 to 250 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as 17 000 cm−1 with respect to the 2P–2S resonance line of the free Mg+ ion and were explained by the shift of this transition as a result of hydration. The spectra also exhibit clear evolution of solvation shell with the first shell closing at n=3, being consistent with the theoretical prediction. The mass spectra of the fragment ions show the existence of two dissociation processes: the evaporation of water molecules and the photoinduced intracluster reaction to produce the hydrated MgOH+ ion, MgOH+(H2O)m. The branching fraction between the two processes depends strongly on the solvent number n and also on the photolysis wavelength. The energetics and the dynamics of the dissociation processes were discussed in conjunction with the results of ab initio calculations.

An ab initio study resulting in a greater understanding of the HSAB principle

Abstract: The quantitative applicability of the hard-soft acid-base (HSAB) principle has been tested. Complexes of HF and Ag+ with several bases (HF, HCl, HBr, H 2 O, H 2 S, H 2 Se, NH 3 , PH 3 , and AsH 3 ) have been studied at the HF, MP2, and QCISD(T) levels with 6-311+G** basis sets. Ab initio pseudopotential calculations have been performed for the molecules containing heavier elements, viz., Br, Se, As, and Ag + . For the hard acid HF, the HSAB principle has been found to be valid even at the HF level. Correlation is important for soft-soft interactions. The maximum hardness principle has been found to be valid for reactions of HF. Reaction of hard acids like H + , Li + , and Na + have also been studied at the MP2/6-311+G** Level. Out of 45 reaction which are not in conformity with the HSAB priniciple.

Calculation of the energies of .pi.* negative ion resonance states by the use of Koopmans' theorem

Abstract: The use of Koopmans'theorem (KT) for the calculation of scaled energies of π * negative ion resonance states has been investigatcd by ab initio molecular theory as a function of basis set. HF/D95v energies calculated for HF/6-31G-optimized geometries have been found to correlate with experimental electron attachment energies for 56 π * negative ion states in 39 alkenes, polyenes, and benzenoid hydrocarbons with a correlation coefficient of 0.989 and with average and largest absolute errors of 0.11 and 0.32 eV, respectively. An equally good correlation was obtained at the same basis set for MP2/6-31G * -optimized geometries. Various factors that influence the quality of KT correlations are analyzed

Ab initio studies of the copper(I) tetramers Cu4X4L4 (X = I, Br, Cl). Effects of cluster structure and of halide on photophysical properties

Abstract: Ab initio calculations at the Hartree-Fock level are described for various compounds of the type Cu 4 X 4 L 4 , in order to gain a better understanding of the rich luminescence behavior of these cuprous halide clusters. The calculations clearly demonstrate a relationship between the Cu-Cu distances (d Cu-Cu ) in these «cubane» type clusters and the energies and distortions (from the ground state) expected for the «cluster-center» (CC) excited states, which arise primarily from a redistribution of charge within the Cu 4 I 4 cluster core

Rotationally inelastic and hyperfine resolved cross sections for OH–H2 collisions. Calculations using a new ab initio potential surface

Abstract: Cross sections and rate constants are presented for the rotational excitation of OH in collision with ortho and para‐H2, using a new ab initio interaction potential [Offer and Van Hemert, J. Chem. Phys. 99, 3836 (1993)]. The cross sections are given at a number of energies and are compared with those calculated using an earlier potential energy surface, and with the available experimental results. A strong oscillatory behavior is found in the cross sections for collisions with ground state para‐H2 which was not apparent in earlier calculations. The oscillatory behavior is very much reduced in collisions with ortho‐H2. Rate constants obtained by averaging the cross sections over a Maxwell–Boltzmann velocity distribution are given at a temperature of 300 K. Expressions for calculating the hyperfine resolved cross sections by transforming the S‐matrices are discussed for the case where H2 is no longer constrained to its rotational ground state, and cross sections for transitions between the hyperfine resolved levels are given for collisions with both para and ortho‐H2.