Wolfgang Förner

Coupled-cluster studies. II. The role of localization in correlation calculations on extended systems

Abstract With the help of a newly developed program for the solution of the coupled-cluster doubles (CCD) equation the use of localized orbitals has been tested

Numerical application of the coupled cluster theory with localized orbitals to polymers. IV. Band structure corrections in model systems and polyacetylene

We present the formalism for the correction of the band structure for correlation effects of polymers in the framework of a localized orbital approximation, using the quasiparticle model. For this purpose we use in an ab initio framework Mo/ller–Plesset perturbation theory in second order, the coupled cluster doubles method, and its linear approximation. The formalism is applied to a water stack and two different forms of a water chain as model systems to test the reliability of the approximations involved. From our previous work we know that, e.g., in polyacetylene difficulties due to the localizability of the canonical crystal orbitals do not arise from the π or π* bands, but from bands of σ symmetry. Thus we concentrate in this work again on polyacetylene as an example of a realistic polymer. We find that the localized orbital approximation is quite useful also in the case of band structure corrections due to correlation effects. However, the coupled cluster calculations, in particular, turn out to be ...

Model calculation of the effect of hydration on the energy band structure of a nucleotide base stack

The energy band structure of the nucleotide base stacks poly C, poly T, poly A, and poly G have been calculated by the ab initio SCF LCAO crystal orbital method. For poly C, model calculations have been performed to investigate the effect of water molecules on its electronic structure. The presence of the water molecules, whose positions have been determined recently by a Monte Carlo simulation technique at T=300 K, causes significant band shifts and together with positive ions could substantially influence the conductive properties of native DNA.

Numerical application of the coupled cluster theory with localized orbitals to polymers. I. Total correlation energy per unit cell

Abstract The orbital invariant Moller-Plesset perturbation theory of second order (LMP2), the coupled cluster doubles theory (CCD) and its linear approximation (L-CCD) are applied to compute total correlation energies per unit cell in different polymers, using localized orbitals. The involved approximations are numerically checked against calculations with the canonical MP2 (CMP2) method. The major aim of this work is to determine numerically, whether the localization properties of Wannier functions are sufficient to make a localized orbital approximation possible or not. Calculations have been performed on hydrogen chains, water chains, polyacetylene, and polyethylene applying different atomic basis sets. Also the correlation energy as function of the geometry of some of the polymers is compared to the CMP2 results. It is found that the localization of the Wannier functions is sufficient for the application of our approximation in non-bonded chains, but not in polymers with chemically bound unit cells.

Analysis of the infrared and Raman spectra of phenylacetic acid and mandelic (2-hydroxy-2-phenylacetic) acid

The structural stability of phenylacetic acid and mandelic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (Np) forms as their lowest energy structures. The observed spectral intensities of the acids were consistent with the Np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The sharpness of the methylenic O-H stretching mode in the IR spectrum of mandelic acid suggests the absence of intermolecular dimerization in the acid which is supported by the observation of no splitting of its CO stretching mode.

Localization of the filled and virtual orbitals in the nucleotide bases

For the four nucleotide bases cytosine, uracil, adenine and guanine both Boys (B) and Edminston-Ruedenberg (ER) localization procedures of the ab initio canonical orbitals have been performed. The results obtained for both σ-π separation and by treating all electrons together show a very good localization for all electrons (one-center lone-pairs and two-center localized orbitals even for π-electrons) and a rather good localization for the virtuals applying both B and ER criteria. The results of the two methods are essentially identical. These results suggest that the application of localized orbitals will open new possibilities for the calculation of correlation in extended systems.

Energies of interactions between purine and pyrimidine bases in B- and Z-DNA

Abstract The results of ab initio SCF LCAO calculations on the dimers of the nucleotide bases for all possible sixteen stacked configurations in B-DNA geometry, on eight stacked conformations in Z-DNA geometry and on the Watson-Crick-type pairs in both DNA configurations are reported. In addition to the SCF interaction energy in the supermolecule approach, the contributions of the dispersion energy have been computed using Londons formula with empirical parameters. In the case of the Watson-Crick-type pairs corrections for the basis-set superposition error are reported, which seem to be negligible for the stacked configuration as a calculation on the adenine dimer shows. The results for the Watson-Crick-type pairs are in good agreement with experimental values and with theoretical ones, obtained by other methods. The stacking energies are mostly attractive but show no significant stabilization of special sequences for B- or Z-DNA. Four dimers are recalculated with the Clementi minimal basis which leads to no considerable qualitative changes in the results. As a better approach to the situation in real DNA we took at least in a qualitative approximation the charge transfer from the sugar phosphate group to the bases into account. Qualitatively this charge transfer destabilizes the pairs to some extent, namely it leads in our approximation to a small repulsion between the stacked bases and lowers the attraction in the Watson-Crick base pairs to some extent.

Quantum and temperature effects on Davydov soliton dynamics. III. Interchain coupling

For pt.II see ibid., vol.5, p.803 (1993). The author shows numerically that Scotts suggestion to use revised parameter values in order to simulate interchain coupling effects on Davydov soliton dynamics with one-chain simulations is correct also for excitations within the mod D2) ansatz state, for which his analytical derivation does not hold. However, in the case of the mod D1) state he finds that the equations of motion for the symmetric mode reduce exactly to the equations for one chain without any renormalization of parameters. Further, he presents numerical simulations for three coupled chains including temperature within different ansatz states and for one chain using Scotts revised parameters. The author found that Davydov solitons should exist in three-chain systems at physiological temperature for reasonable parameter values.

Influence of substitutional impurities on soliton dynamics in trans‐polyacetylene

For different chains of trans‐polyacetylene with various substituents the equations of motion of the coupled electron–phonon system are integrated within the Su–Schrieffer–Heeger model. NH+, N, and O+ as isoelectronic substitutions for a CH group as well as the effect of an NH and a CO group are investigated. The calculations for the time evolution of an end‐generated kink show that neutral solitons can pass a nitrogen atom and an oxygen ion, but not an NH+, NH, or CO group. The negatively charged soliton is not able to pass any of the investigated substitutions. The CO unit, which is of special interest in the light of recent experimental results for acetylene–CO–copolymers with similar properties as trans‐polyacetylene, represents a trap for both neutral and negatively charged kinks and a repulsive barrier for a positively charged kink. The limitations of the soliton model are discussed.

The energy band structure of polyfluoroethylene: Influence of chemical substitution and conformation

Abstract Ab initio SCF LCAO crystal orbital band structure calculations for six different poly(fluoroethylene)s derived from trans-polyethylene replacing stepwise the hydrogen atoms by fluorine atoms and for eighteen helical configurations of poly(tetrafluoroethylene) are reported. The dependence of the energy band structure on the basis set has been investigated. The theoretical density of electronic- state spectra is discussed and compared with the experimental ESCA spectrum of poly(tetrafluoroethylene), showing a good agreement. The potential energy surface as a function of the rotation angle about the CC bond agrees well with the experimental data.