Showing papers on "Ab initio quantum chemistry methods published in 2000"

A Road Map for the Calculation of Molecular Binding Energies

Abstract: During the past decade dramatic progress has been made in calculating the binding energies of molecules. This is the result of two advances reported in 1989: an accurate method for solving the electronic Schrodinger equation that is applicable to a broad range of moleculesthe CCSD(T) methodand families of basis sets that systematically converge to the complete basis set limitthe correlation consistent basis sets. The former provides unprecedented accuracy for the prediction of a broad range of molecular properties, including molecular binding energies. The latter provides a means to systematically approach the complete basis set limit, i.e., the exact solutions of approximations to the Schrodinger equation. These two advances combined with a thorough analysis of the errors involved in electronic structure calculations lead to clear guidelines for ab initio calculations of binding energies, ranging from the strong bonds derived from chemical interactions to the extremely weak binding due to dispersion int...

The Magnitude of the CH/π Interaction between Benzene and Some Model Hydrocarbons

Abstract: High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and hydrocarbon molecules (methane, ethane, ethylene, and acetylene). Intermolecular interaction energies were calculated from extrapolated MP2 interaction energies at the basis set limit and CCSD(T) correction terms. The calculated benzene−methane interaction energy (−1.45 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The benzene−methane complex prefers a geometry in which the C−H bond points toward the benzene ring. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the complex (0.85 kcal/mol) is repulsive. The large gain of the attraction energy (−2.30 kcal/mol) by electron correlation correction indicates that dispersion interaction is the major source of the attraction. Although the electrostatic energy (−0.25 kcal/mol) is small, a highly ori...

Material Design of Half-Metallic Zinc-Blende CrAs and the Synthesis by Molecular-Beam Epitaxy

Abstract: A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Origin of the Attraction and Directionality of the NH/π Interaction: Comparison with OH/π and CH/π Interactions

Abstract: High-level ab initio calculations were carried out to evaluate the interaction between the π face of benzene and ammonia as a model of NH/π interaction. The intermolecular interaction energy was calculated from the extrapolated MP2 interaction energy at the basis set limit and a CCSD(T) correction term. The calculated interaction energy (−2.22 kcal/mol) is considerably smaller than that of the hydrogen bond between waters. The monodentate complex is slightly more stable than the bidentate and tridentate complexes. The potential energy surface is very flat near the minimum, which shows that the major source of the attraction is a long-range interaction. The HF interaction energy of the monodentate complex (0.13 kcal/mol) is repulsive. The large gain in the attraction by electron correlation correction (−2.36 kcal/mol) indicates that the dispersion interaction is significantly important for the attraction. The electrostatic energy (−1.01 kcal/mol) is also important for the attraction. The benzene−water (OH/...

Analytical calculation of nuclear magnetic resonance indirect spin–spin coupling constants at the generalized gradient approximation and hybrid levels of density-functional theory

Abstract: A fully analytical implementation of the nuclear magnetic resonance (NMR) indirect nuclear spin–spin coupling constants at the density-functional theory (DFT) level is presented. The implementation involves all four contributions of the nonrelativistic Ramsey theory: The dia- and para-magnetic spin–orbit contributions as well as the paramagnetic Fermi-contact and spin–dipole contributions. Three different exchange-correlation functionals—LDA (local density approximation), BLYP (Becke–Lee–Yang–Parr), and B3LYP (hybrid BLYP)—are tested by comparison with experiment and high-level ab initio calculations for a series of molecules containing first-row elements. All three levels of theory represent a significant improvement on restrictred Hartree–Fock (RHF) theory in the sense that the RHF instability problems are absent in DFT. Also, there is a steady improvement in the quality of the calculated spin–spin couplings in the sequence LDA, BLYP, and B3LYP. For the first-row molecules investigated by us, the hybrid...

Infrared Spectra of H+(H2O)5-8 Clusters: Evidence for Symmetric Proton Hydration

Abstract: Protonated water clusters, H+(H2O)n (n = 5−8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 ± 20 K. Recorded absorption bands at the frequency range of 2700−3900 cm-1 are attributed to the free- and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H+(H2O)5-8 isomers are close in energy, with the excess proton either localized on a single water molecule, yielding H3O+(H2O)n-1, or equally shared by two molecules, yielding H5O2+(H2O)n-2. Systematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H5O2+(H2O)4 was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm-1. The existence of five-membe...

Quantum confinement in germanium nanocrystals

Abstract: The electronic structure of Ge nanocrystals is studied using a sp3 tight binding description. Analytical laws for the confinement energies, valid over the whole range of sizes, are derived. We validate our results with ab initio calculations in the local density approximation for smaller clusters. Comparing to experimental data, we conclude that, similar to the case of silicon: (a) the blue-green photoluminescence (PL) of Ge nanocrystals comes from defects in the oxide and (b) the size dependent PL in the near infrared probably involves a deep trap in the gap of the nanocrystals. We predict that the radiative lifetimes remain long in spite of the small difference (0.14 eV) between direct and indirect gaps of bulk Ge.

Planar Hexacoordinate Carbon: A Viable Possibility

Abstract: The viability of molecules with planar hexacoordinate carbon atoms is demonstrated by density-functional theory (DFT) calculations for CB62–, a CB6H2 isomer, and three C3B4 minima. All of these species have six π electrons and are aromatic. Although other C3B4isomers are lower in energy, the activation barriers for the rearrangements of the three planar carbon C3B4minima into more stable isomers are appreciable, and experimental observation should be possible. High-level ab initio calculations confirm the DFT results. The planar hexacoordination in these species does not violate the octet rule because six partial bonds to the central carbons are involved.

Computational determination of equilibrium geometry and dissociation energy of the water dimer

Abstract: The equilibrium geometry and dissociation energy of the water dimer have been determined as accurately as technically possible. Various quantum chemical methods and high-quality basis sets have been applied—that is, at the level of a nearly complete basis—and both the intermolecular separation and the deformation of the donor and acceptor molecules have been optimized at the level of CCSD(T) theory (coupled-cluster theory with singles and doubles excitations plus a perturbation correction for connected triples). It is found at the CCSD(T) level that the monomer deformation in the dimer amounts to 86% of the deformation computed at the MP2 level (second-order Moller-Plesset perturbation theory) and that the core/valence electron correlation effects at the CCSD(T) level amount to 80% of the same effects at the MP2 level. The equilibrium O···O distance is determined as Re=291.2±0.5 pm and the equilibrium dissociation energy as De=21.0±0.2 kJ mol−1, with respect to dissociation into two isolated water molecules at equilibrium. Accounting for zero-point vibrational energy, the theoretical prediction for the dissociation energy becomes D0=13.8±0.4 kJ mol−1, a result which is open to direct experimental verification.

Convergence testing of the analytic representation of an ab initio dipole moment function for water: Improved fitting yields improved intensities

Abstract: In general, when computing intensities for polyatomics, one has to interpolate the dipole moment function obtained from ab initio calculations. For some high overtones of the water molecule, the computed intensities can be very sensitive to the way in which the interpolation is done. Our previous analytic representation [H. Partridge and D. W. Schwenke, J. Chem. Phys. 106, 4618 (1997)] was not adequate. We show that stable results can be obtained, and these results are in much improved agreement with experiment. We also test the importance of core electron correlation on intensities, and find the effect to be negligible. Of the existing water dipole moment functions in the literature, the present one is the most accurate.

Ab initio phonon dispersions of wurtzite AlN, GaN, and InN

Abstract: Phonon excitations play an important role in electronic transport, nonradiative electron-relaxation processes, and other properties of interest for materials characterization, device engineering, and design. We have calculated the phonon dispersions and density of states for wurtzite AlN, GaN, and InN using state-of-the-art density-functional perturbation theory. The calculations are in good agreement with the existing experimental data for zone-center modes and predict the full phonon dispersions throughout the Brillouin zone. In particular, it is found that the three-phonon decay of the LO phonon in two acoustic phonons is not allowed in GaN and InN, since the LO frequency is much larger than the acoustic frequencies over the entire spectrum. The substantial potential of the group-III nitrides and their alloys for applications in optoelectronic and high speed devices has attracted a great deal of interest. AlN, GaN, and InN have direct energy gaps which span a substantial range, from the visible to the ultraviolet region of the spectrum. Consequently, their alloys have direct gaps that can be tuned to any value within this range simply by varying the alloy composition. This tunability offers many possibilities for device engineering. In particular, InGaN alloys were used in the realization of light emitting diodes and laser diodes operating in the blue and UV spectral region. Under ambient conditions, the III nitrides crystallize in a hexagonal, wurtzite ~2H! structure, although thin films having a cubic, zincblende ~3C! structure have also been grown. 1 Properties of interest for device engineering and design, such as electronic transport, nonradiative electron relaxation processes, lattice specific heat, etc., are strongly influenced by phonon excitations. Furthermore, a number of nondestructive experimental techniques of sample characterization, for instance, Raman spectroscopy or IR reflectivity, involve phonon measurements. A characterization of the phonon dispersions and densities of states for the group-III nitrides is therefore desirable. However, since it is very challenging to grow single crystals of suitable size for neutron-scattering experiments, there are no experimental data for the phonon dispersions of these compounds. Only very recently have the phonon density of states for AlN and GaN been obtained from time-of-flight neutron spectroscopy using bulk powders. 2,3 In addition, numerous studies of the zone-center phonons in GaN and AlN films have been conducted using Raman and IR spectroscopy. Due to the lack of latticematched substrates, these samples are affected by the high density of defects and strain present in the films and it is therefore unclear how well these data represent the true bulk values. The least studied of the three nitrides is InN for which there are only few Raman studies. 4,5

Representation of the 6D potential energy surface for a diatomic molecule near a solid surface

Abstract: An efficient method is proposed to construct the six-dimensional Potential Energy Surface (PES) for diatomic molecule-surface interactions from low dimensional cuts obtained in ab initio calculations. The efficiency of our method results from a corrugation-reducing procedure based on the observation that most of the corrugation in a molecule-surface PES is already embedded in the atom-surface interactions. Hence, substraction of the latter leads to a much smoother function which makes accurate interpolations possible. The proposed method is a general one and can be implemented in a systematic way for any system. Its efficiency is illustrated for the case of H2/Pd(111) by using recent ab initio data. We report also the results of very stringent checks against ab initio calculations not used in the interpolation. These checks show the high accuracy of our method.

An investigation of the F+H2 reaction based on a full ab initio description of the open-shell character of the F(2P) atom

Abstract: Expanding on an earlier Communication [M. H. Alexander, H.-J. Werner, and D. E. Manolopoulos, J. Chem. Phys. 109, 5710 (1998)], we present here the full framework for the quantum treatment of reactions of the fluorine atom with molecular hydrogen. This involves four potential energy surfaces (PESs) and two, coordinate-dependent spin–orbit interaction terms, all of which were fitted to the results of ab initio calculations. Quantum scattering calculations, based on a time-independent method formulated in hyperspherical coordinates, were carried out to determine initial and final state-resolved reactive cross sections, for reaction of F in its ground (2P3/2) and excited (2P1/2) spin–orbit state with H2 in j=0 and j=2(pH2) and j=1(oH2). The overall reactivity of the excited state of F, which can occur only through nonadiabatic transitions, is found to be small, at most 25% of the reactivity of the ground spin–orbit state, which is adiabatically allowed. In addition, when compared with results of earlier calculations, based on a single, electronically adiabatic, PES, our calculations show that even fine details of the dynamics of the F+H2 reaction will be well described by calculations on a single PES. The contribution of the excited spin–orbit state can be seen most clearly in the formation of HF products in the v=3 vibrational manifold, which are nearly thermoneutral (or even slightly endoergic) in the reaction of ground-state F atoms. The cross section for the near resonant electronic-rotational process [F*+H2(j=0)→F+H2(j=2)] is found to be large, in confirmation of earlier work.

Ab initio calculations for a hypothetical material: Silicon nanotubes

Abstract: Electronic and structural properties of a hypothetical material, silicon nanotubes, are examined through first-principles calculations based on density functional theory. Even considering that Si nanotubes have never been observed, this paper attempts to establish the theoretical similarities between Si and C, like band structures and density of states, as well as the main differences, especially associated with cohesive energies. The band-structure calculations for silicon nanotubes show that, similar to carbon structures, depending on their chiralities, they may present metallic (armchair) or semiconductor (zigzag and mixed) behaviors.

An Absolute Sodium Cation Affinity Scale: Threshold Collision-Induced Dissociation Experiments and ab Initio Theory

Abstract: Threshold collision-induced dissociation of Na+(L) with xenon is studied using guided ion beam mass spectrometry. The ligand L includes ethene, benzene, phenol, ammonia, acetaldehyde, acetone, and N,N-dimethylformamide. In all cases, the primary product formed corresponds to endothermic loss of the neutral ligand and the only other product observed is the result of ligand exchange processes to form NaXe+. The cross-section thresholds are interpreted to yield 0 and 298 K bond energies for Na+−L after accounting for the effects of multiple ion−molecule collisions, internal energy of the reactant ions, and dissociation lifetimes. Ab initio calculations at several levels of theory compare favorably to the experimentally determined bond energies for these and previously studied systems, L = Ar, CO, dimethyl ether, H2O, methylamine, imidazole, dimethoxyethane, and several alcohols. Combined, these ligands cover a very wide range in binding energies, and thereby help to establish an absolute scale for sodium cat...

Electronic and vibronic contributions to two-photon absorption of molecules with multi-branched structures

Abstract: The electronic and vibronic contributions to the two-photon absorption of a series molecules with multi-branched structures have been studied using ab initio response theory. The results indicate that the electronic coupling between different branches alone cannot explain the experimental finding of a strong enhancement of the two-photon absorption cross section over the single branch structure, whereas it is predicted that vibronic contributions can play an important role in this respect. It is shown that for multi-branched molecules the use of circularly polarized light can increase the two-photon absorption cross section by a factor of 1.5 over linearly polarized light excitation.

High-Level Ab Initio Calculations of Dihydrogen-Bonded Complexes

Abstract: High-level ab initio calculations have been performed on dihydrogen-bonded complexes with hydrogen fluoride (HF) as a proton-donating molecule and simple molecules as proton-acceptors (CH4, SiH4, BeH2, MgH2, LiH, and NaH). MP4(SDQ)/6-311++G** and QCISD(T)/6-311++G** results show that H-bond energies for such systems are significant. For example, the H-bond energy is −11.9 kcal/mol for the LiH···HF complex at the QCISD(T)/6-311++G** level of theory; the basis set superposition error (BSSE) was included, and the geometry of the complex was optimized at the QCISD/6-311++G** level. The relationships between the geometrical parameters of these complexes are in good agreement with those obtained from the bond valence model. The BSSE is taken into account in all levels of calculations. A comparison of the results of the calculations shows that the MP2/6-311++G** level of theory is sufficient for a description of dihydrogen-bonded complexes. Additionally, Bader's theory is included in the analysis of the investig...

Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters

Abstract: X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor inter...

Accuracy of atomization energies and reaction enthalpies in standard and extrapolated electronic wave function/basis set calculations

Abstract: The accuracy of standard ab initio wave-function calculations of atomization energies and reaction enthalpies has been assessed by comparing with experimental data for 16 small closed-shell molecules and 13 isogyric reactions. The investigated wave-function models are Hartree–Fock (HF), Moller–Plesset second-order perturbation theory (MP2), coupled-cluster theory with singles and doubles excitations (CCSD) and CCSD with perturbative triple-excitation corrections [CCSD(T)]; the one-electron basis sets used are the correlation-consistent cc-pVxZ and cc-pCVxZ basis sets with cardinal numbers x=D, T, Q, 5, and 6. Results close to the basis-set limit have been obtained by using two-point extrapolations. In agreement with previous studies, it is found that the intrinsic error of the CCSD(T) method is less than chemical accuracy (≈4 kJ/mol) for both atomization energies and reaction enthalpies. The mean and maximum absolute errors of the best CCSD(T) calculations are 0.8 and 2.3 kJ/mol for the atomization energi...

Absolute binding energies of alkali-metal cation complexes with benzene determined by threshold collision-induced dissociation experiments and ab initio theory

Abstract: The sequential bond dissociation energies (BDEs) of the mono- and bis-benzene complexes with alkali metal cations (Li+, Na+, K+, Rb+, and Cs+) are determined experimentally by collision-induced dissociation (CID) with Xe in a guided ion beam mass spectrometer and theoretically by ab initio calculations. The kinetic energy dependence of the CID cross sections are analyzed to yield 0 and 298 K bond energies for (C6H6)x-1M+−C6H6 (x = 1−2) after accounting for the effects of the internal energies of the reactant ions, the multiple collisions of the ions with xenon, and the dissociation lifetimes of the ionic complexes. Ab initio binding energies are calculated at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G* level and corrected for zero-point energies (ZPE) and basis set superposition errors (BSSE). The theoretical BDEs are in reasonably good agreement with the experimentally determined 0 K bond energies when full electron correlation is included (for Li+, Na+, and K+) but differ appreciably when effective c...

All-electron ab-initio molecular dynamics

Abstract: We present an all-electron implementation of the Gaussian and augmented plane wave density functional method (GAPW method), which allows ab-initio density functional calculations for periodic and non-periodic systems. The GAPW method uses a Gaussian basis set to expand the Kohn–Sham orbitals, whereas an augmented plane wave basis set is introduced as an auxiliary basis set to expand the electronic charge density. The results of the all-electron calculations for a representative set of small molecules are reported to demonstrate the accuracy and reliability of the GAPW method. Furthermore, its performance is shown for some larger systems, including calculations on unbranched alkane chains up to n-C100H202 with 1804 basis functions and a fully hydrated crystalline RNA duplex (sodium guanylyl-3′-5′-cytidine nonahydrate) with 368 atoms and 3168 basis functions. Finally, as a first test an all-electron ab-initio molecular dynamics (MD) run was performed for 32 water molecules in a simple cubic box under ambient conditions. A standard single processor workstation (IBM 397) was used for all the presented calculations.

Nuclear spin–spin coupling constants from regular approximate relativistic density functional calculations. II. Spin–orbit coupling effects and anisotropies

Abstract: Based on our recently published two-component relativistic formulation of the nuclear spin–spin coupling hyperfine terms, we present a full implementation into the Amsterdam Density Functional program. The scalar relativistic code has been extended to include the relativistic analogue of the spin–dipole operator in the coupling calculations, which can now in addition be based on two-component spin–orbit coupled Kohn–Sham orbitals. One-bond coupling constants for some plumbanes are in good agreement with experiment, slightly improving the scalar relativistic values. Coupling constants and anisotropies for the XF (X=Cl, Br, I) and TlX (X=F, Cl, Br, I) series are compared to experimental data and for ClF additionally to recently published ab initio calculations. The spin–dipole term contributes largely to the coupling constants in XF. Spin–orbit effects are essential for the TlX couplings, where they can yield the most important contributions. In addition, data is reported for the benchmark systems ethane, ethene, and ethyne.

Nuclear Scalar Spin−Spin Couplings and Geometries of Hydrogen Bonds

Abstract: Ab initio calculations of the scalar coupling constants 1J15N-1H ≡ JNH and 2J15N···15N ≡ JNN of the N−H···N hydrogen bonds in the anion [C⋮15N···L···15N⋮C]- (1), L = H, D, and of the cyclic hydroge...