Showing papers on "Ab initio published in 1982"

Approximate evaluations of the electrostatic free energy and internal energy changes in solution processes

Abstract: A recently proposed procedure for introducing solvent effects in the molecular hamiltonian of a solute is here re-elaborated to get approximate solutions of the corresponding classical electrostatic problem. The basic feature of the original procedure, i.e. the direct utilization of a quantum-mechanical ab initio description of the solute charge distribution in the “continuum” solution model, with cavities of arbitrary shape, is maintained. The meaning of supplementary assumptions introduced in classical calculation 0is discussed, and a comparison is made with analogous evaluations obtained with other approaches

THEORY OF STATIC STRUCTURAL PROPERTIES, CRYSTAL STABILITY, AND PHASE TRANSFORMATIONS: APPLICATION TO Si AND Ge

Abstract: We demonstrate that not only the static structural properties but also the crystal stability and pressure-induced phase transformations in solids can be accurately described employing an ab initio pseudopotential method within the local-density-functional formalism. With the use of atomic numbers of constituent elements and a subset of crystal structures as the only input information, the calculated structural properties of Si and Ge are in excellent agreement with experiment.

The ab initio calculation of the vibrational‐rotational spectrum of triatomic systems in the close‐coupling approach, with KCN and H2Ne as examples

Abstract: A Hamiltonian for the vibration‐rotation motions of atom–diatom systems is derived in body‐fixed coordinates and a method for its solution as a close‐coupled secular problem is formulated. The radial coordinate is expanded in Morse oscillator functions. Calculations on KCN and H2Ne are presented. For KCN the neglect of Coriolis interactions is found to have little effect. Extensions of the method to problems in more dimensions are suggested.

Ab Initio Force Constants of GaAs: A New Approach to Calculation of Phonons and Dielectric Properties

Abstract: It is shown that self-consistent calculations of the electronic charge density in large periodic cells containing a single displaced atom provide all the information needed for ab initio determination of force constants, phonon dispersion curves, effective charges, and the static dielectric constant. Results are presented for GaAs.

Polyatomic canonical variational theory for chemical reaction rates. Separable‐mode formalism with application to OH+H2→H2O+H

Abstract: A formalism for the application of variational transition‐state theory and semiclassical vibrationally adiabatic transmission coefficients to bimolecular reactions involving an arbitrary number of atoms is presented. This generalizes previous work on atom–diatom reactions. We make applications in this paper to the reactions OH+H2→H2O+H and OH+D2→HDO+D using the Schatz–Elgersma fit to the Walch–Dunning ab initio potential energy surface. For both reactions we find large differences between conventional and variational transition‐state theory and large effects of anharmonicity on the calculated rate constants. The effect of reaction‐path curvature on the calculated transmission coefficients and rate constants is also large. The final calculated values of the kinetic isotope effects are in good agreement with experiment at high temperature but too large at room temperature.

Stabilization of methyl anions by first‐row substituents. The superiority of diffuse function‐augmented basis sets for anion calculations

Abstract: The entire set of methyl anions, XCH 2 - , substituted by first-row substituents, Li, BeH, BH 2 , CH 3 , NH 2 , OH, and F, was examined at various ab initio levels. Diffuse orbital-augmented basis sets, such as 4-31+G and 6-31+G ∗ , are needed to describe the energies of these anions adequately. Estimates of proton affinities are further improved by second-order Moller-Plesset (MP2) electron correlation corrections, but relative energies are less affected. The methyl group in the ethyl anion is destabilizing, the amino substituent is borderline, but all other groups are stabilizing. Very large π effects are exhibited by BH 2 and BeH groups; inductive stabilization by the electronegative F and OH groups is less effective. Lithium also is stabilizing, but the best singlet geometry of CH 2 Li - is not planar. A planar CH 2 Li - triplet with a π 1 configuration may be lower in energy.

An efficient algorithm for calculating ab initio energy gradients using s, p Cartesian Gaussians

Abstract: A method is presented for the calculation of analytical first derivatives of the two electron integrals over s‐ and p‐type Cartesian Gaussian basis functions. Formulas are developed for derivatives with respect to the positions of the nuclei and basis functions (for use in geometry optimization) and with respect to the exponents of the primitive Gaussians (for use in basis set optimization). Full use is made of the s = p constraint on the Gaussian exponents. Contributions from an entire shell block are computed together and added to the total energy derivative directly, avoiding the computation and storage of the individual integral derivatives. This algorithm is currently being used in the ab initio molecular orbital program gaussian 80.

Theory of lattice-dynamical properties of solids: Application to Si and Ge

Abstract: It is demonstrated that the ab initio pseudopotential theory within the local-density-functional formalism provides an accurate theoretical framework for the study of lattice-dynamical properties of solids. With the use of atomic numbers and masses of constituent elements and the crystal structure as the only input information, the calculated phonon frequencies and mode-Gr\"uneisen parameters at $\ensuremath{\Gamma}$ and $X$, the third-order force constant for LTO ($\ensuremath{\Gamma}$), the shear modulus, and the zone-center TA [110] velocity are all in excellent agreement with experiment. Comparison with other microscopic calculations is made.

Features of the photochemically active state surfaces of azobenzene

Abstract: To discuss the main features of the potential energy curves of azobenzene along the two possible isomerization paths ab initio and cofiguration-interaction (CI) computations have been performed for the cis and trans isomers and for the intermediate geometries along the rotational and the inversion isomerization paths. From the expression of the excited states, we propose a correlation diagram and potential energy curves for the ground and lowest excited states. These curves, integrated with the available experimental data, provide a basis for interpreting the photochemical and photophysical properties of azobenzene.

Electronic Structure of Polymers

Abstract: Publisher Summary This chapter discusses the development of microscopic models of the electronic structure of polymers in general and of conjugated organic polymers in particular The underlying physical model—namely, the one-particle picture, is being widely used in this field and is considered basically correct On a practical level, this means that periodic models are usually constructed, which justify using the terminology of energy–band theory With refinements of calculation tools, self-consistent calculations at even the ab initio level have become widespread The chapter focuses on methodological problems related to the treatment of the electronic structure of polymers, such as the boundary conditions and problems related to the large size of these systems, ab initio techniques, and a symmetry dilemma connected with the quasidegeneracy of partially filled bands The results of the application of these and semi-empirical techniques to the carbon atomic chain to (CH) x and a series of related polymers, polydiacetylene (PDA), and some further conjugated polymers, are compared with experimental results Most of the discussion is based on one-dimensional (1D) model

Ab initio effective Hamiltonian study of the electronic properties of conjugated polymers

Abstract: The valence effective Hamiltonian technique is applied to a series of polymers to compute ionization potentials, bandwidths, and band gaps. The polymers considered represent systems of interest to the conducting polymers area and include various derivatives of polyacetylene and polyphenylene, polydiacetylene, polyacene, polybenzyl, and polyyne. The theoretical results for relative ionization potentials are in excellent agreement with available experimental estimates, as well as with the observed behavior of the electrical conductivity of these systems on exposure to weak (I2) versus strong (AsF5) electron acceptors. The bandwidths of the highest occupied band show a qualitative correlation to the conductivities achieved with acceptor doping. Band gaps for the planar systems considered are also in good agreement with experiment.

Tautomerization of formamide, 2-pyridone, and 4-pyridone: an ab initio study

Abstract: The tautomerism of formamide, 2-pyridone, and 4-pyridone has been investigated by ab initio calculations using minimal, extended, and polarization basis sets. When the effects of geometry optimization, polarization functions, correlation energy (estimated by second-order Merller-Plesset perturbation theory), and zero-point vibration energy are combined, the following theoretical estimates are obtained: formimidic acid is 12 kcal/mol less stable than formamide, 2-pyridone is 0.3 kcal/mol more stable than 2-hydroxypyridine and 4-hydroxypyridine is 2.4 kcal/mol more stable than 4-pyridone. Only the 2-pyridone tautomerism has been observed directly in the gas phase, and theory is in good agreement with all three experimental values (0.3 f 0.3,O.l & 0.6,0.6 f 0.1 kcal/mol). In the case of 4-pyridone, the theoretical value may be closer to the actual tautomerization energy than the 7 kcal/mol in favor of hydroxypyridine obtained from indirect experiments. For the heterocycles, relative geometries of tautomers optimized with a minimal basis or semiempirical methods are as satisfactory as structural changes obtained at extended basis set levels. Relative tautomerization energies are reproduced well with the minimal or extended bases, while absolute tautomerization energies require consideration of polarization functions, correlation energy, and zero-point vibration. Tautomerism such as displayed by pyridone/hydroxypyridine plays a role in many areas of chemistry and biochemistry: e.g., the rationalization of structures, properties, and reactivities in heterocyclic chemistry;'*2 concepts and probes of aromaticity;3 measures of intrinsic stabilities vs. solvent mechanisms of enzymatic catalysis and receptor interactions;6 and possibly even mutations during DNA replicati~n.~.~ Investigations of tautom- erism of 2-pyridone date from 1907.8 Most studies since then have dealt with the equilibrium in liquid media,'v9 where the pyridone tautomer is preferred by a factor of 1000. X-ray crystallography shows that pyridone is also favored in the solid.'*I2 The dominance of the pyridone tautomer in solution, neat liquid, and solids has been shown to be the result of strong solvent effects, ion binding, and self- association^.'^^^^^^^'^ In contrast, recent IR and UV measurements have established that the two tautomers are nearly equal in energy when unassociated in the vap~r.~,'~J~ Similar gas-phase tautomerizations have since been investigated for a number of lactam/lactim pairs by using IR,I9 UV,zo pho- toelectron,21*22 ion cyclotron resonan~e,~~-~~ and mass spectros- copy.26*27 All of these gas-phase equilibria show marked dif- ferences from solution data.'*2,9,'3-'7*z8 Numerous theoretical studies with almost every available method have attempted to reproduce the tautomerization energy for pyridone/hydroxypyridine and similar heterocyclic systems2- Simulations of hydrogen bonding and solvent interactions re- produce qualitatively the shift in the equilibrium toward pyridone in condensed However, quantitative agreement with the tautomerization energy in the vapor has been difficult to obtain. Geometry optimization, basis-set flexibility, correlation energy, and zero-point vibration have been recognized as important contributors to these and related45-55 isomerization reactions. In this paper, we report an extensive series of ab initio computations on formamide, 2-pyridone, 4-pyridone, and their tautomers that take these factors into account. Method Ab initio calculations were carried out with the GAUSSIANBO series of programss6 by using minimal (STO-3G)," extended(3-21G and 6- 31G),5*~~~ and polarization (6-31G*)" basis sets. The extended basis sets are of the split-valence. type, and the polarization basis set is an extended basis augmented by a shell of six Cartesian d-type Gaussians on each non-hydrogen atom. Energies were calculated in the Hartree-Fock (HF) approximation, and correlation effects were estimated via second-order

Nonlinear optical properties of organic crystals with hydrogen‐bonded molecular units: The case of urea

Abstract: The validity of simpler additive models, accounting for nonlinear optical properties of van der Waals crystals in terms of independent molecular contributions is questionable in the case of more associated molecular structures such as examplified by the hydrogen‐bonded crystal lattice of urea. A number of one‐electron properties of single isolated urea molecules are first considered by means of a finite‐field perturbated LCAO Hartree–Fock model implemented at both semiempirical (INDO) and ab initio levels of approximation with different basis set extensions. Charge distribution, dipole (μ), multipoles, linear (α), quadratic (β), and cubic hyperpolarizabilities γ are computed and analyzed. The α tensor is shown to be quasi‐isotropic in the molecular plane while the β tensor exhibits a stronger anisotropy and a prevailing βzzz coefficient (z is the molecular symmetry axis). The negative value of the product βzzzμz is interpreted by a decrease of the dominant excited state dipole as compared to that of the g...

Theory of ab initio pseudopotential calculations

Abstract: The ab initio norm-conserving pseudopotential is generated from a reference atomic configuration in which the pseudoatomic eigenvalues and wave functions outside the core region agree with the corresponding ab initio all-electron results within the density-functional formalism. This paper explains why such pseudopotentials accurately reproduce the all-electron results in both atoms and in multiatomic systems. In particular, a theorem is derived to demonstrate the energy- and perturbation-independent properties of ab initio pseudopotentials.

Electronic distributions within protein phenylalanine aromatic rings are reflected by the three-dimensional oxygen atom environments.

Abstract: The atomic environments of 170 phenylalanine-residue aromatic rings from 28 protein crystal structures are transformed into a common orientation and combined to calculate an average three-dimensional environment. The spatial distribution of atom types in this environment reveals a preferred interaction between oxygen atoms and the edge of the planar aromatic rings. From the difference in frequency of interaction of oxygen atoms with the edge and the top of the ring, an apparent net free energy difference of interaction favoring the edge of the ring is estimated to be about -1 kcal/mol (1 cal = 4.184 J). Ab initio quantum mechanical calculations, performed on a model consisting of benzene and formamide, indicate that the observed geometry is stabilized by a favorable enthalpic interaction. Although benzene rings are considered to be nonpolar, the electron distribution is a complex multipole with no net dipole moment. The observed interaction orientation frequencies demonstrate that these multipolar electron distributions, when occurring at the short distances encountered in densely packed protein molecules, are significant determinants of internal packing geometries.

Theoretical studies of the chemisorption of hydrogen on copper

Abstract: The chemisorption of H2 on Cu(100) is treated using a many‐electron embedding theory in which the copper lattice is modeled as a 38‐atom cluster. Ab initio valence (4s) CI calculations carried out on a local electronic subspace permit an accurate description of bonding at the surface. Dissociated hydrogen is found to bind exotherimically to the (100) surface at several stable adsorption sites in fourfold, bridge, and atop atom H positions. Calculated H2 binding energies are in the range 13–22 kcal/mol. However, high energy barriers to dissociation of 35–40 kcal/mol exist due to the repulsion of molecular H2 by the surface and the difficulty of stretching H2 significantly above the surface. A characteristic of stretched or dissociated H2 is the occurrence of H levels well up into the Cu 4s band from 5.5 to 7.8 eV below EF.

Unique determination of the He2 ground state potential from experiment by use of a reliable potential model

Abstract: Measured backward glory oscillations of integral 4He2 and 3He2 scattering cross sections are evaluated by use of an improved semiclassical backward glory formula yielding the energy dependence of the s phases, which allows the calculation of the He2 potential in the region 1.83–2.12 A via Miller’s semiclassical inversion method. A physically realistic two‐parameter potential model, which uses all ab initio data available with sufficient accuracy, is fitted to the inversion result, giving in a unique way the entire helium pair potential function. A well with a depth of 10.74 K at 2.975 A is obtained, which supports a bound state for 4He2 very near to the dissociation limit. Via construction, the potential model also gives results for individual interaction energy terms in the symmetry adapted perturbation scheme for the He2 interaction. Calculations with the determined potential reproduce the various experimental data available for helium. The best ab initio He2 potentials available today converge towards that potential.

Relativistic ab initio molecular structure calculations including configuration interaction with application to six states of TlH

Abstract: A system for the inclusion of spin-orbit coupling along with moderate scale CI in calculations for molecules containing very heavy elements is demonstrated. In this effective potential procedure rigorous ab initio spin-orbit integrals are computed and added to the conventional integral set after the SCF and integral transformation steps of the calculation. This avoids the use of complex coefficients in the integral transformation and yet includes spin-orbit corrections on an equal footing with electron correlation. The diagonalization of the resulting complex CI plus SO matrix requires only about twice the time of a real CI diagonalization. Our present calculations on the two lowest 0/sup +/ and 1 states and the lowest 0/sup -/ and 2 states of TlH indicate that this procedure allows adequate flexibility in the electronic coupling, resulting in bonding curves which are in good agreement with the experimentally established curves. The results also help to understand and to confirm previously conjectural interpretations of other spectral data.

How well does the Hartree–Fock model predict equilibrium geometries of transition metal complexes? Large‐scale LCAO–SCF studies on ferrocene and decamethylferrocene

Abstract: Large scale ab initio LCAO–SCF calculations performed on ferrocene show that the Hartree–Fock model is unable to account for the experimentally observed metal to ring distance. The present results, using basis sets of better than triple zeta quality, show that both the equilibrium geometry and the orbital energies have converged already at the double zeta level (metal‐ring distance;1.89 A, i.e., 15% larger than the experimental value of 1.65 A). A comparative calculation on decamethylferrocene yields essentially the same results. These findings raise some doubts as to the adequacy of the Hartree–Fock model for predictions of equilibrium geometries of transition metal complexes in general.

Quantum chemical studies of a model for peptide bond formation: formation of formamide and water from ammonia and formic acid

Abstract: The S/sub N/2 reaction between ammonia and formic acid has been studied as a model reaction for peptide bond formation using the semiempirical MNDO and ab initio molecular orbital methods. Two reaction mechanisms have been examined, i.e., a stepwise and a concerted reaction. The stationary points of each reaction including intermediate and transition states have been identified and free energies have been calculated for all geometry optimized reaction species to determine the thermodynamics and kinetics of each reaction. The stepwise mechanism was found to be more favorable than the concerted one by both MNDO and ab initio calculations. However, the ab initio method predicts both mechanisms to be fairly competitive with free energies of activation of about 50 kcal/mol. Despite excellent agreement between both methods in the calculated entropies and thermal energies, the minimum basis set character of MNDO leads to values of free energy of activation much higher than those obtained by the ab initio method. The basis set dependence and effect of correlation of the computed ab initio results and the relative effects of polarization and correlation were also investigated by using a number of basis sets up to 6-31G** and estimates of correlation energy by Moller-Plesset perturbationmore » theory up to fourth order. Correlation energy was found to ba a significant factor in the stabilization of transition states.« less

Statistical‐diabatic model for state‐selected reaction rates. Theory and application of vibrational‐mode correlation analysis to OH(nOH)+H2(nHH)→H2O+H

Abstract: The state‐selected reaction rates OH(nOH = 0,1)+ H2(nHH = 0,1)→H2O+H are calculated by an extension of variational transiton state theory. The reactant vibrational modes are assumed to correlate diabatically with generalized normal modes of a generalized activated complex. Using the Walch‐Dunning‐Schatz‐Elgersma ab initio potential energy surface, the theory predicts that excitation of H2 is 19–68 times more effective than excitation of OH in promoting reaction at 300 K, where the range of values corresponds to different possible assumptions about the quantal effects on reaction‐coordinate motion. These values are in much better agreement with the experimental value (about 100) than is a calculation based on the conventional transition state, which yields 2×104.

Ab initio studies of structural features not easily amenable to experiment. 23. Molecular structures and conformational analysis of the dipeptide N‐acetyl‐N′‐methyl glycyl amide and the significance of local geometries for peptide structures

Abstract: The molecular structures of four conformations of N‐acetyl–N′‐methyl glycyl amide were refined by geometrically unconstrained ab initio gradient relaxation on the 4–21G level. The most stable form I contains a seven‐membered ring closed by hydrogen bonding. A second local minimum II is less than 1 kcal/mol above I and represents the fully extended form with a five‐membered hydrogen bonded ring. The two other minima refined, III and IV, are open forms which are 4–5 kcal/mol less stable than I. The refined geometries make it possible to estimate the significance of local geometries, in contrast to standard geometry, in the various conformations. It is found that bond distances in different conformations can vary by up to 0.02 A, and important backbone bond angles can vary by up to 7°. Except for the symmetrical form II, small deviations from amide planarity (H–N–C = 0 angles of 3–10°) are the rule, even though the equilibrium structure of the unperturbed amide group in 4–21G space is planar. It can be concl...

Quantum dynamics of the van der Waals molecule (N2)2: An ab initio treatment

Abstract: Starting with an available ab initio N2–N2 potential, which favors a crossed equilibrium structure for the (N2)2 dimer with well depth De=122 cm−1, Re=3.46 A, and barriers to internal rotations of 25 and 40 cm−1, we calculate the bound rovibrational states of this dimer for J=0, 1, and 2. This is done by solving a secular problem over the exact (rigid monomer) Hamiltonian including centrifugal distortions and Coriolis interactions, using a product basis of radial (Morse oscillator) functions and angular momentum eigenfunctions. The full permutation‐inversion symmetry of the system, in relation to the nuclear spin coupling, is used in order to simplify the calculations and to derive selection rules for IR absorption. We find that the (N2)2 dimer has a large number of bound rovibrational states (92 already for J=0). These are analyzed by correlation with rigid molecule (harmonic oscillator/rigid rotor) results, on the one hand, and with the states of two freely rotating N2 monomers, on the other, and by plo...

Estimates for systematic empirical corrections of consistent 4–21G ab initio geometries and their correlations to total energy group increments

Abstract: The structural parameters of the completely relaxed 4–21G ab initio geometries of more than 30 basic organic compounds are compared to experimental results. Some ranges for systematic empirical corrections, which relate 4–21G bond distances to experimental parameters, are associated with total energy increments. In general, for the currently feasible comparisons, the following corrections can be given which relate calculated distances to experimental r g parameters and calculated angles to r s -structures For CC single bond distances, deviations between calculated and observed parameters ( r g ) are in the ranges of −0.006(2) to −0.010(2) A for normal or unstrained hydrocarbons; −0.011(3) to −0.016(3) A for cyclobutane type compounds; and +0.001(5) to +0.004(4) A for CH 3 conjugated with CO. For CO single bonds the ranges are −0.006(9) to +0.002(3) A for CO conjugated with CO; and −0.019(3) to −0.027(9) A for aliphatic and ether compounds. A very large and exceptional discrepancy exists for the highly strained ethylene oxide, r s — r e = −0.049(5) A and in CH 3 OCH 3 and C 2 H 5 OCH 3 the r s — r e differences are −0.029(5), −0.040(10) and −0.025(10) A. Some of these discrepancies may also be due to deficiencies of the microwave substitution method caused by atomic coordinates close to inertial planes. For CN bonds, two types of NCH 3 corrections are from +0.005(6) to −0.006(6) and from −0.009(2) to −0.014(6) A; and the range for NCO is +0.012(3) to +0.028(4) A. For isolated CC double bonds the range is + 0.025(2) to +0.028(2) A. For conjugated CC double bonds the correction is less positive (+0.014(1) A for benzene). For CO double bonds the corrections are −0.004(3) to +0.003(3) A. For bond angles of type HCH, CCH, CCC, CCO, CCO, OCO, NCO and CCC the corrections are of the order of magnitude about 1–2° (or better). Angles centered at heteroatoms are less accurate than that, when hydrogen atoms are involved. Differences in HOC and NHC angles were found in a range of −2.3(5)° to −6.2(4)°.

Vibrational frequencies from anharmonic ab initio/empirical potential energy functions. III. Stretching vibrations of hydrogen cyanide and acetylenes

Abstract: Stretching vibrational states are calculated for various isotopes of hydrogen cyanide, acetylene, fluoroacetylene, chloroacetylene and cyanoacetylene by diagonalizing an approximate vibrational hamiltonian which neglects the anharmonic coupling between stretching and bending motions. Empirically corrected ab initio potential energy functions are constructed from which the lower lying vibrational transition frequencies are obtained with an overall accuracy of about 20 cm −1 . The influence of electron correlation on different potential energy terms is investigated for hydrogen cyanide and acetylene by means of the coupled electron pair approximation (CEPA) within the framework of the self-consistent electron pairs theory (SCEP). Equilibrium geometries are estimated for the substituted acetylenes and quartic and sextic centrifugal distortion constants are calculated for all molecules under investigation.