P. Otto

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

On the electronic structure and conduction properties of aperiodic DNA and proteins. I. Strategy and methods of investigations

Abstract The possibilites for applying the ab initio matrix block negative factor counting technique (NFC) for non-periodic nucleotide base stacks and for 4- and 5-component aperiodic model proteins are described. It is outlined how also correlation effects could be taken into account in an approximate way. After a brief summary of the basic equations of the NFC method the inverse iteration technique to study the Anderson localization of the wavefunctions belonging to the different energy levels is shortly reviewed. Also the calculation of hopping probabilities between different localization sites is sketched. Finally, the possibilites are outlined for the experiment verification of hopping conduction in proteins and (in a minor extent) in DNA, assuming the generation of free charge carriers in them through charge transfer.

On the electronic structure and conduction properties of aperiodic DNA and proteins. IV. Electronic structure of aperiodic proteins

Abstract Electronic density of states (DOS) curves of single stranded periodic and aperiodic DNA base stacks (in B conformation) and of a fragment of a human gene, obtained with the help of ab initio matrix block negative factor counting (NFC) method are presented. The calculated large values of band gap obtained for these systems rule out the possibility of intrinsic conduction in them. It is also found that in contrast to the corresponding period systems, the peaks in the DOS curves of aperiodic systems are broader. The probable effects of these changes in the DOS on the possibility of hopping conduction in DNA are also discussed.

Calculation of ab initio dynamic hyperpolarizabilities of polymers

The coupled Hartree–Fock (CHF) equations in second order are derived to calculate dynamic polarizabilities and hyperpolarizabilities for infinite periodic chains. The analytical expressions for the second derivatives of the perturbed crystal orbitals with respect to the quasimomentum k are developed. The first and second derivatives are required on behalf of the definition of the perturbation operator describing the effect of the time-dependent electric field on the electronic structure of the polymer. The computer program has been applied to calculate the tensor elements of the second-harmonic generation and the optical rectification for the model chain poly(water) and the conjugated π-electron system poly(carbonitrile), respectively. The CHF-results are compared with uncoupled Hartree–Fock (UCHF) calculations.

Relativistic all-electron Hartree–Fock–Dirac calculation of a quasi one-dimensional chain of selenium atoms

Abstract The relativistic molecular program MOLFDIR has been extended to calculate the energy band structure and electronic structure of infinite periodic systems on the basis of the ab initio four-component Hartree–Fock–Dirac equations. The first application has been performed for a quasi-one-dimensional linear chain of selenium atoms. The basis set for the polymeric calculation has been generated by fitting Gaussian functions to the numerical relativistic results of the atom with the help of the adaptive non-linear least-square algorithm. The relativistic energy band structure is compared with the ab initio non-relativistic Hartree–Fock result employing the relativistic basis set.

Investigation of the interaction between molecules at medium distances: II. Perturbational and MCF calculations with directly integrated potentials and in the monopole approximation

Abstract The calculated interaction energies of two HF and two H 2 O molecules are presented using both mutually consistent field (MCF) and perturbation theoretical schemes. Instead of the monopole approximation, the direct integration of the potentials was used in these calculations. It is found that the results in both schemes agree quantitatively with the supermolecule results calculated with the same basis set. Also presented are the interaction energies of two three molecular systems, 2HF plus H 3 O + and 2HF plus H + . These calculations were made using the monopole approximation in all the three schemes supermolecule, MCF, and perturbation method. The MCF results indicate that the interaction energy between the two HF molecules depends strongly on the position of the third partner. On the other hand, one finds that the perturbation theoretical scheme is not able to describe this effect. In addition to the above, the dispersion interaction energy has been calculated for all the investigated systems.

On the electronic structure and conduction properties of aperiodic DNA and proteins. III. The band structures of periodic polypeptides

Abstract In this paper the calculation of the energy band structure of periodic polypeptides using the ab initio Hartree-Fock crystal orbital method is described. Results are discussed for the twenty homopolypeptides in the β-pleated sheet configuration and for several periodic systems assuming the α-helix structure. The negative factor method in its matrix block form is used to provide density of states curve for periodic two-component polypeptides. The resulting properties of the band structure suggest that homopolypeptides and periodic more-component polyamino acids are in themselves insulators, but may become weak semiconductors if free charge carriers are generated in their valence or conduction bands, respectively, through charge transfer.

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.

Investigation of the interaction between molecules at medium distances. III. SCF LCAO MO supermolecule, perturbational and MCF calculations for two and

Abstract The interaction energies of 2HF, 2H2O, 2CH2O and 2HCONH2 molecules at medium intermolecular distances have been calculated with the aid of the mutually consistent field and perturbational schemes. Instead of using the direct integration method the Coulomb potentials have been represented by a point charge distribution. The MCF model gives in all cases better results as compared to the supermolecule calculations than the perturbational ones. Also reported are the results for interaction energies of three systems consisting of three molecules 2HF + H3O+, 2H2O + H3O+ and 3H2O, respectively, in the supermolecule, perturbational and MCF approach with the aid of potentials calculated by point charge distributions. The MCF results are in better agreement with the SM ones and show the influence of the third partner molecule on the interaction of the two other ones, while on the other hand this effect cannot be described in the perturbational scheme. Comparing the computer times needed in the three approaches, respectively, one can see that for small molecules using a medium size basis set the supermolecule calculations need less time than the two other schemes while for larger molecules ano even small molecules with an extended basis set the order is reversed and the MCF method needs less computer time than the perturbation theoretical scheme.

Improved band structures of some homopolypeptides with aliphatic side chains and of the four nucleotide base stacks : estimation of their fundamental gap

Abstract The ab initio band structures of seven similar homopolypeptides (polyglycine, polyalanine, polyvaline, polyleucine, polyisoleucine, polyserine and polythreonine) in the β-pleated sheet conformation were calculated using Clementis double-zeta basis set. Similar computations were executed for the four nucleotide base stacks. To obtain an estimate of the physically interesting fundamental gaps of these systems, their Hartree-Fock (HF) gaps were multiplied by a scale factor. This factor was established by dividing the correlation corrected quasi-particle (QP) gap by the HIP gap in the cases of polyglycine and a cytosine stack for which more accurate large-scale calculations have been performed previously. Finally an attempt was made to estimate a realistic gap value for each of the 11 investigated quasi-one-dimensional systems using different physical arguments. The gap values are between about 7.8 and about 8.5 eV for the homopolypeptides and between about 4.6 and about 5.9 eV for the nucleotide base stacks.