Showing papers on "Ab initio published in 1993"
TL;DR: In this paper, the authors present an ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local density approximation.
Abstract: We present ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local-density approximation at each molecular-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using subspace alignment. This approach avoids the instabilities inherent in quantum-mechanical molecular-dynamics calculations for metals based on the use of a fictitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows us to perform simulations over several picoseconds.
32,798 citations
TL;DR: In this article, quasi-relativistic energy-adjusted pseudopotentials for the elements of groups 13-17 up to atomic number 53 (I) are presented together with corresponding energy-optimized valence basis sets.
Abstract: Quasi-relativistic energy-adjusted ab initio pseudopotentials for the elements of groups 13–17 up to atomic number 53 (I) are presented together with corresponding energy-optimized valence basis sets. Test calculations for atomic excitation and ionization energies show the reliability of the derived pseudopotentials and basis sets.
2,483 citations
TL;DR: The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed in this article, where equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient-corrected DFT methods, and also by the ab initio methods Hartree-Fock, second order Mo/ller-Plesset, and quadratic configuration interaction with single and double substitutions (QCISD).
Abstract: The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed. Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient‐corrected DFT methods, and also by the ab initio methods Hartree–Fock, second‐order Mo/ller–Plesset, and quadratic configuration interaction with single and double substitutions (QCISD). The standard 6‐31G* basis set was used for orbital expansion, and self‐consistent Kohn–Sham orbitals were obtained by all DFT methods, without employing any auxiliary fitting techniques. Comparison with experimental results shows the density functional geometries and dipole moments to be generally no better than or inferior to those predicted by the conventional ab initio methods with this particular basis set. The density functional vibrational frequencies compare favorably with the ab initio results, while for at...
1,736 citations
Cray1
TL;DR: Three-center approximations to the four-center integrals occurring in ab initio LCAO calculations are investigated in this paper, where significant gains in computer time can be obtained without sacrificing accuracy, if a suitable expansion basis is chosen.
1,474 citations
TL;DR: A systematic procedure for generating optimized nonlocal pseudopotentials by minimizing the high Fourier components of the pseudo-wave-functions with the constraints of normalization and continuity of first and second derivatives of the wave function at the core radius is introduced.
Abstract: The purpose of the present work is to obtain a better atomic pseudopotential with respect to convergence and computational efficiency while retaining reasonable transferability in the context of electronic-structure calculations for solids using a plane-wave basis set. We introduce a systematic procedure for generating optimized nonlocal pseudopotentials by minimizing the high Fourier components of the pseudo-wave-functions with the constraints of normalization and continuity of first and second derivatives of the wave function at the core radius. This is based on the recent ideas of Rappe et al. (RRKJ) [Phys. Rev. B 41, 1227 (1990)], but overcomes certain difficulties which we have found with the RRKJ scheme. For computational efficiency this optimized nonlocal pseudopotential is transformed into a Kleinman-Bylander (KB) form. To ensure the transferability we first compare the logarithmic derivative of the all-electron wave function with that of the final KB form of the optimized nonlocal pseudopotential over a wide range of energies. We then test the KB form of the potential in a number of atomic environments. The structural properties of ZnS are calculated to demonstrate the reliability of our optimized nonlocal separable ab initio pseudopotential and its total-energy convergence.
904 citations
TL;DR: In this article, a vibrational assignment for the -d0 molecule is facilitated by the availability of spectral data for five different isotopomers, including S-methyl-N, N-dimefhylthiocarbamate, (CH3)2NC(O)SCH3, and its isotopomer, S-d3, n-d6 and N -d9, for the gas and liquid.
839 citations
TL;DR: In this article, the optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree-Fock and second order perturbation theory (for n=2-4) levels of theory with an augmented correlation consistent double zeta basis set.
Abstract: The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size The predicted R0(O–O) for the ring hexamer is less than 002 A shorter than the interoxygen separation in ice Ih Furthermore, the trends in the harmonic vibrational frequencies sugge
730 citations
TL;DR: Improved energy-adjusted quasirelativistic pseudopotentials for lanthanoid atoms with fixed valency were presented and tested in molecular calculations for CeO, CeF, EuO, GdO, YbO, and YbF as discussed by the authors.
Abstract: Improved energy-adjusted quasirelativistic pseudopotentials for lanthanoid atoms with fixed valency are presented and tested in molecular calculations for CeO, CeF, EuO, GdO, YbO, and YbF. The pseudopotential calculations treat the lanthanoid 4f shell as part of the core and yield accurate estimates for average bond lengths, vibrational frequencies and dissociation energies of all states belonging to a superconfiguration. Information for each individual state of the considered superconfiguration may be obtained from subsequent ligand field model calculations. The results of this combined pseudo-potential and ligand field approach (PPLFT) are compared to more accurate calculations with ab initio pseudopotentials that include the lanthanoid 4f orbitals explicitly in the valence shell and to available experimental data.
642 citations
TL;DR: In this article, an energy-adjustable pseudopotential scheme for element 105 (hahnium, Ha) with corresponding energy-optimized valence basis sets is presented.
Abstract: Nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials are presented for element 105 (hahnium, Ha) together with corresponding energy-optimized valence basis sets. The method of energy adjustment of pseudopotentials is extended to a two-component formalism and to multiconfiguration wave functions. The accuracy of the pseudopotential scheme is demonstrated by a comparison of atomic valence-only results to corresponding all-electron data. Atomic multiconfiguration self-consistent field and multireference configuration interaction calculations for M and M[sup +] (M = Nb, Ta, Ha) are compared with available experimental data. Corresponding molecular calculations, which included spin-orbit coupling, have been performed for the low-lying states of HaO and are compared to the results from corresponding calculations of the lighter homologs NbO and TaO. 41 refs., 1 fig., 8 tabs.
631 citations
TL;DR: Atomic charges were obtained from ab initio molecular orbital calculations using a variety of procedures to compare them and assess their utility, and the ability of the charges to reproduce electrostatic potentials was examined.
Abstract: Atomic charges were obtained from ab initio molecular orbital calculations using a variety of procedures to compare them and assess their utility Two procedures based on the molecular orbitals were examined, the Mulliken population analysis and the Weinhold–Reed Natural Population Analysis Two procedures using the charge density distribution were included; the Hirshfeld procedure and Bader's Atoms in Molecules method Charges also were derived by fitting the electrostatic potential (CHELPG) and making use of the atomic polar tensors (GAPT) The procedures were first examined for basis set independence, and then applied to a group of hydrocarbons The dipole moments for these molecules were computed from the various atomic charges and compared to the total SCF dipole moments This was followed by an examination of a series of substituted methanes, simple hydrides, and a group of typical organic compounds such as carbonyl derivatives, nitriles, and nitro compounds In some cases, the ability of the charges to reproduce electrostatic potentials was examined © John Wiley & Sons, Inc
618 citations
TL;DR: An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn-Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step.
Abstract: An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.
TL;DR: The ability to predict chemical shifts in proteins from known or test structures opens new avenues to structure refinement or determination, especially for condensed systems.
Abstract: Recent theoretical developments permit the prediction of 1H, 13C, 15N, and 19F nuclear magnetic resonance chemical shifts in proteins and offer new ways of analyzing secondary and tertiary structure as well as for probing protein electrostatics. For 13C, phi, psi torsion angles dominate shielding for C alpha and C beta, but the addition of hydrogen bonding and electrostatics gives even better accord with experiment. For 15NH, side chain (chi 1) torsion angles are also important, as are nearest neighbor sequence effects, whereas for 1HN, hydrogen bonding is particularly significant. For 19F, weak or long-range electrostatic fields dominate 19F shielding nonequivalencies. The ability to predict chemical shifts in proteins from known or test structures opens new avenues to structure refinement or determination, especially for condensed systems.
TL;DR: In this article, the results of valence-only self-consistent field calculations on Hg n+ (n = 0, 1, 2) and HgH n+(n = 1, 0, 0) using nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials for Hg are compared with corresponding all-electron values from non-relative (Hartree-Fock) and relativistic (Dirac Fock) atomic calculations.
Abstract: The results of valence-only self-consistent field calculations on Hg n+ (n = 0, 1, 2) and HgH n+ (n = 0, 1) using nonrelativistic and quasirelativistic energy-adjusted ab initio pseudopotentials for Hg are compared with corresponding all-electron values from nonrelativistic (Hartree-Fock) and relativistic (Dirac-Fock) atomic as well as from nonrelativistic (Hartree-Fock) and quasirelativistic (Hartree-Fock with no-pair Hamiltonian) molecular calculations. The accuracy of the energy-adjusted ab initio pseudopotential scheme, e.g., the reproduction of the major relativistic effects, is demonstrated both for the atom and the molecule. Correlation effects are included in the quasirelativistic pseudopotential studies by means of large-scale configuration interaction calculations. The quasirelativistic pseudopotential results obtained in the intermediate coupling scheme are in excellent agreement with available experimental data.
TL;DR: The calculated structural properties of the wurtzite and rocksalt phases are in good agreement with experiment, indicating that the Hartree-Fock linear-combination-of-atomic-orbitals method can reliably predict quite small energy differences between different densities or crystal structures of a nonmetallic solid.
Abstract: The total energy of ZnO as a function of unit cell volume has been calculated for the zinc-blende, wurtzite, and rocksalt structures by the ab initio all-electron periodic Hartree-Fock linear-combination-of-atomic-orbitals method using a large Gaussian basis set that was variationally optimized for the solid state. Extensive convergence tests with respect to cutoffs of the real-space Coulomb and exchange series were carried out to ensure that the calculations were performed with sufficient precision. The calculated structural properties (equilibrium lattice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment, as is the transition pressure between them (8.57 GPa versus 9--9.5 GPa experimentally), indicating that the method can reliably predict quite small energy differences between different densities or crystal structures of a nonmetallic solid. The calculated valence-band structure was also in satisfactory agreement with experiment. Detailed analysis of the charge-density distribution supports the expected picture of a transition from mixed ionic-covalent bonding in the tetrahedrally coordinated structures to predominantly ionic bonding in the high-pressure phase.
TL;DR: In this article, the temperature dependence of the proton transfer equilibrium constants for approximately 80 pairs of bases ranging in proton affinity from N 2 to tert-butylamine has been examined.
Abstract: The temperature dependence of the proton transfer equilibrium constants for approximately 80 pairs of bases ranging in proton affinity from N 2 to tert-butylamine has been examined. These data provide the basis for formulation of a revised gas-phase proton affinity scale which now appears to have a firm basis. Excellent agreement with appearance energy determinations of proton affinities as well as ab initio calculated values is obtained. An important finding of this work is that the value of ΔH f o for the tert-butyl cation must be significantly higher than that derived from appearance energy measurements by Traeger which had formed the basis for the proton affinity assignment for isobutene, an important reference point in the proton affinity scale
TL;DR: In this paper, the structural properties of Si4, Si6 and Si7 clusters were investigated using surface plasmon-polariton enhanced Raman spectroscopy, and it was shown that Si4 is a planar rhombus, and Si6 is a distorted octahedron, while Si7 is a pentagonal bipyramid.
Abstract: SMALL clusters of silicon, containing between 2 and 100 atoms, have been studied extensively both because of their intrinsic interest from the point of view of chemical structure and bonding and because of the potential technological applications of cluster-assembled materials1–3. Ab initio quantum-chemical calculations predict that very small clusters should have structures markedly different from that of the crystalline phase4–12. Experiments on two- (ref. 13), three- (ref. 14) and four-atom15 clusters have allowed some comparison with theoretical predictions, but structural information on larger clusters has been only indirect16. Here we report on structural studies of size-selected Si4 , Si6 and Si7 clusters prepared and isolated by low-energy deposition into a solid nitrogen matrix. Surface plasmon-polariton enhanced Raman spectroscopy17–20 yields well resolved vibrational spectra for each of these clusters in which the vibrational frequencies agree well with those predicted for optimized structures calculated by ab initio methods. We confirm that Si4 is a planar rhombus, and find that Si6 is a distorted octahedron and Si7 a pentagonal bipyramid.
TL;DR: A calculation strategy is presented that allows the calculation of dimer structures without resolving ther ambuguity by additional experiments (like asymmetric labeling), and employs a molecular dynamic‐based simulated annealing approach to minimize a traget function.
Abstract: The structure determination of symmetric dimers by NMR is impeded by the ambiguity of inter- and intramonomer NOE crosspeaks. In this paper, a calculation strategy is presented that allows the calculation of dimer structures without resolving the ambiguity by additional experiments (like asymmetric labeling). The strategy employs a molecular dynamics-based simulated annealing approach to minimize a target function. The experimental part of the target function contains distance restraints that correctly describe the ambiguity of the NOE peaks, and a novel term that restrains the symmetry of the dimer without requiring the knowledge of the symmetry axis. The use of the method is illustrated by three examples, using experimentally obtained data and model data derived from a known structure. For the purpose of testing the method, it is assumed that every NOE crosspeak is ambiguous in all three cases. It is shown that the method is useful both in situations where the structure of a homologous protein is known and in ab initio structure determination. The method can be extended to higher order symmetric multimers.
TL;DR: A symmetry conserving method which allows for efficient structural searches and optimizations in spaces with preselected symmetry groups and is used to investigate MgSiO[sub 3], a perovskite, the marjor Earth-forming mineral phase which exists particularly in the lower mantle.
Abstract: We report the development of an ab initio constant pressure extended molecular dynamics method with variable cell shape. This is a symmetry conserving method which allows for efficient structural searches and optimizations in spaces with preselected symmetry groups. We have used it to investigate ${\mathrm{MgSiO}}_{3}$, a perovskite, the marjor Earth-forming mineral phase which exists particularly in the lower mantle. We predict its structural behavior up to pressures which exceed the highest values reached in this region.
TL;DR: Comparison with initial-state results reveals an enhanced screening at the surface, which is even remarkably different for the two atoms forming the surface dimer.
Abstract: Typically surface core-level shifts (SCLS) of clean surfaces are explained in the initial-state model, thus ignoring the screening of the photon-induced hole We will show that this approach is not valid for the (001) surfaces of Si and Ge Using ab initio density-functional theory we calculate the SCLS from differences of total energies of slabs containing excited atoms at different positions at the surface and in the bulk Comparison with initial-state results reveals an enhanced screening at the surface, which is even remarkably different for the two atoms forming the surface dimer
TL;DR: In this paper, a detailed account focusing on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity is presented.
Abstract: This Account focuses on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity The authors have an enormous body of basic chemical reactivity data for M[sup +] and a growing body of data for M[sup 2+] and neutral M Sophisticated experiments can control the kinetic energy and the electronic state of M[sup +] reactants One can study the reactivity of Fe[sup +] in the 3d[sup 6]4s, high-spin ground state, the 3d[sup 7], high-spin first excited state, or the 3d[sup 6]4s, low-spin second excited state The authors have learned to follow the elimination of H[sub 2] and C[sub 2]H[sub 6] from bimolecular Ni[sup +](n-butane) complexes in real time, on a 50-ns time scale In M[sup +] reactions, the authors can control the kinetic energy and the electronic state of the reactants A key advantage in interpreting these results is that one understands the electronic structure of the bare metal atom reactants very well Solution-phase chemists might well question the relevance of atomic species with genuine 1+ or 2+ charges and no ligands or solvent to the [open quotes]real world[close quotes] of organometallic chemistry Yet connections surely exist, as witnessedmore » by the fact that Rh and Ir atoms are unusually reactive in all phases Theoretical chemists are beginning to provide a conceptual framework that will unify seemingly diverse branches of experimental chemistry Of necessity, the ab initio quantum chemist works on model transition metal systems, draws experimental evidence from all available sources, and tries to abstract from the calculations what is robust and common to all phases Gas-phase metal atoms are idealized model systems well matched to the capabilities of modern theory Many new conceptual insights in the next 10 years will come from careful analysis of ab initio wave functions informed by incisive gas-phase experiments 30 refs, 5 figs, 1 tab« less
TL;DR: The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H2 reaction and suggests that the transition state region of the F - H2 potential energy surface has finally been understood quantitatively.
Abstract: The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.
TL;DR: In this article, the authors present high-resolution optical and microwave spectra of the benzene-ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benene plane and undergoes free or nearly free internal rotation.
Abstract: AMINES have long been characterized as amphoteric (acting as both donor and acceptor) in terms of their hydrogen-bond interactions in the condensed phase. With the possible exception of (NH_3)_2, however, no gas-phase complexes exhibiting hydrogen-bond donation by ammonia, the ‘simplest amine’, have been observed. Here we present high-resolution optical and microwave spectra of the benzene–ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benzene plane and undergoes free or nearly free internal rotation. In the vibrationally averaged structure, the C_3 symmetry axis of NH_3 is tilted by about 58° relative to the benzene C_6 axis, such that the ammonia protons interact with the benzene π-cloud. Our ab initio calcula-tions predict a 'monodentate' minimum-energy structure, with very low barriers to rotation of ammonia. The larger separation of the two molecular components, and the smaller dissociation energy, relative to the benzene–water dimer reflect the weak hydrogen-bond donor capability of ammonia, but the observed geometry greatly resembles the amino–aromatic interaction found naturally in proteins.
TL;DR: A first principles calculation of lattice dynamical properties of diamond using density-functional perturbation theory together with plane-wave expansion and nonlocal pseudopotentials and the validity of the ab initio calculation for describing properties beyond the harmonic approximation is tested.
Abstract: We present a first-principles calculation of lattice dynamical properties of diamond. Our calculations have been performed using density-functional perturbation theory together with plane-wave expansion and nonlocal pseudopotentials. As a first step we have evaluated the equilibrium structure of diamond via the minimization of the total energy. Then, harmonic phonon dispersion curves and phonon eigenvectors have been evaluated within the linear-response framework. As a by-product of the calculation we have also obtained the internal-strain parameter. Furthermore, we have also tested the validity of the ab initio calculation for describing properties beyond the harmonic approximation. Using the quasiharmonic approximation we have calculated the thermal expansion coefficient and the mode Gr\"uneisen parameter dispersion curves. Where experimental data are available, good agreement is found with our theoretical predictions.
TL;DR: In this article, a self-consistent Kohn-Sham (KS) theory with local density approximation (LDA) and gradient-corrected exchange-correlation (BLYP) functionals are used to optimize the geometries and calculate the frequencies of methane, acetylene, ethylene, and benzene.
Abstract: Self-consistent Kohn-Sham (KS) theory with local density approximation (LDA) and gradient-corrected exchange-correlation (BLYP) functionals are used to optimize the geometries and calculate the frequencies of methane, acetylene, ethylene, and benzene. Large basis sets are employed with accurate numerical quadrature. The predictions are compared with ab initio calculations and with experiment. KS (BLYP) bond lengths are a little long with the result that the stretching frequencies are too small, but good predictions are obtained for bending frequencies. In particular, the ω 14 B 2u mode of benzene is predicted to within 10 cm -1 , unlike all of the ab initio methods which apparently require multiconfiguration methodology
TL;DR: The well-known trio of IR bands (A, B, C) at approximately 2800, approximately 2400, and approximately 1700 cm-1, typical for strong H complexes in vapors, liquids, and solids, was also found in CD3CN adsorption on Bronsted sites of HZSM-5 and HY zeolites as discussed by the authors.
Abstract: The well-known trio of IR bands (A, B, C) at approximately 2800, approximately 2400, and approximately 1700 cm-1, typical for strong H complexes in vapors, liquids, and solids, is also found in CD3CN adsorption on Bronsted sites of HZSM-5 and HY zeolites. The observed CN frequencies in interaction with different Bronsted and Lewis sites of these zeolites are quantitatively reproduced by ab initio 3-21 g calculations combined with the frequency scaling procedure
TL;DR: In this paper, the authors evaluated the importance of d functions on hydrogen and f functions on carbon, nitrogen, and oxygen on the potential energy hypersurface of glycine using a large basis set TZ2P+f.
Abstract: Ab initio quantum mechanical methods, including the self-consistent field (SCF), single and double excitation configuration interaction (CISD), the single and double excitation coupled cluster (CCSD), and the single, double, and perturbative triple excitation coupled cluster [CCSD(T)] have been applied to five C s conformers and four of their C l counterparts on the potential energy hypersurface of glycine. A large basis set TZ2P+f designated H(5s2p1d/3s2p1d) and C,N,O(10s6p2d1f/5s3p2d1f) was chosen to evaluate the importance of d functions on hydrogen and f functions on carbon, nitrogen, and oxygen
TL;DR: In this article, the authors investigated the magnitude of and how to eliminate the basis set superposition errors at different levels of theory and showed that at the Hartree-Fock level the superposition error is insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study.
Abstract: The water dimer has been studied by accurate ab initio calculations. The main purpose of the calculations was to investigate the magnitude of, and how to eliminate the basis set superposition errors at different levels of theory. At the Hartree–Fock level the superposition errors are insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study. At the correlated level superposition errors are still significant even for very large basis sets, and the standard counterpoise technique leads to overcorrection. The most important result of the present study is that the local correlation methods gives essentially the correct result for the correlation contribution to the association energy even with modest basis sets. The association energy at the MP4(SDQ) level is predicted to be 4.8 kcal/mol. The correlation contribution to the association energy is 1.2 kcal/mol which can be decomposed into an attractive intermolecular contribution of 1.8 kcal/mol and a repulsive intramolecular contribution of 0.6 kcal/mol. Ionic terms contribute about 30% to the dispersion force at the equilibrium distance. If the effect of triple substitutions is taken into account the association energy is estimated to be around 5.1 kcal/mol.
TL;DR: The first parameter-free calculations of self-diffusion constants in silicon are reported, with the self-interstitial mechanism dominating over the contribution of the other mechanisms.
Abstract: We report the first parameter-free calculations of self-diffusion constants in silicon. We have computed diffusion constants for the defect-mediated mechanisms using the local-density approximation in combination with ab initio molecular-dynamics simulations and obtained the diffusion constant for the concerted exchange mechanism from earlier results by Pandey and Kaxiras. We obtain diffusion constants in the range of the experimental values, with the self-interstitial mechanism dominating over the contribution of the other mechanisms.
01 Jan 1993
TL;DR: In this article, the authors present a survey of the state-of-the-art work in the field of non-rigid molecular structures and energy properties, focusing on vibrational and rotational spectra of nonrigid molecules.
Abstract: From the beginnings of modern chemistry, molecular structure has been a lively area of research and speculation. For more than half a century spectroscopy and other methods have been available to characterize the structures and shapes of molecules, particularly those that are rigid. However, most molecules are at least to some degree non-rigid and this non-rigidity plays an important role in such diverse areas as biological activity, energy transfer, and chemical reactivity. In addition, the large-amplitude vibrations present in non-rigid molecules give rise to unusual low-energy vibrational level patterns which have a dramatic effect on the thermodynamic properties of these systems. Only in recent years has a coherent picture of the energetics and dynamics of the conformational changes inherent in non-rigid (and semi-rigid) molecules begun to emerge. Advances have been made in a number of different experimental areas: vibrational (infrared and Raman) spectroscopy, rotational (microwave) spectroscopy, electron diffraction, and, most recently, laser techniques probing both the ground and excited electronic states. Theoretically, the proliferation of powerful computers coupled with scientific insight has allowed both empirical and ab initio methods to increase our understanding of the forces responsible for the structures and energies of non-rigid systems. The development of theory (group theoretical methods and potential energy surfaces) to understand the unique characteristics of the spectra of these floppy molecules has also been necessary to reach our present level of understanding. The thirty chapters in this volume contributed by the key speakers at the Workshop are divided over the various areas. Both vibrational and rotational spectroscopy have been effective at determining the potential energy surfaces for non-rigid molecules, often in a complementary manner. Recent laser fluorescence work has extended these types of studies to electronic excited states. Electronic diffraction methods provide radial distribution functions from which both molecular structures and compositions of conformational mixtures can be found. Ab initio calculations have progressed substantially over the past few years, and, when carried out at a sufficiently high level, can accurately reproduce (or predict ahead of time) experimental findings. Much of the controversy of the ARW related to the question of when an ab initio is reliable. Since the computer programs are readily available, many poor calculations have been carried out. However, excellent results can be obtained from computations when properly done. A similar situation exists for experimental analyses. The complexities of non-rigid molecules are many, but major strides have been taken to understand their structures and conformational processes.
TL;DR: In this paper, the authors used the 3-21G basis set to locate transition structures for concerted and stepwise mechanisms of the Diels-Alder and [2+2] dimerizations of 1,3-butadiene.
Abstract: Ab initio CASSCF calculations with the 3-21G basis set have been fixed to locate transition structures for concerted and stepwise mechanisms of the Diels-Alder and [2+2] dimerizations of 1,3-butadiene. The butadiene-ethylene reaction was also studied with six-orbital/six-electron CASSCF calculations and the 3-21G and 6-31G * basis sets. The CASSCF calculations appear to slightly overestimate the stabilities of biradicals relative to concerted mechanisms, based upon comparisons with experimerital data and expectations about correlation energies of different species. Energies were also evaluated with QCISD(T)/6-31G * calculations