Janos Ladik

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.

The electronic structure of DNA related periodic polymers

Results of ab initio LCAO Hartree–Fock crystal orbital calculations using a minimal atomic basis set are reported for the single stranded periodic B‐DNA models of cytosine, thymine, adenine, and guanine stacks and two polynucleotides with adenosine and thymidine as repeating unit, respectively. Further, the energy band structures of two poly(base pairs), poly(adenine–thymine), and poly(guanine–cytosine), representing a simple model of the B‐DNA double helix are discussed.

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 ...

New implementation of a program to calculate correlated band structures of polymers: an application to the band structure of polyparaphenylene (PPP)

A new program has recently been implemented with the aim of extending quasi‐particle (QP) band structure calculations to polymers with larger unit cells. The theoretical background is briefly reviewed and the new algorithm described, which has been optimized for machines with vector processors. To illustrate the usefulness of this technique, calculations have been performed on polyparaphenylene (PPP) using a double‐zeta basis set. The calculated QP band gap between the valence and conduction bands is 2.3 eV, which compares favorably with the experimental value of 2.8 eV. The self‐consistent field (SCF) result with the same double‐zeta basis set is 8.7 eV.

A non-empirical molecular orbital study on the relative stabilities of adenine and guanine tautomers

The relative stabilities of a series of adenine and guanine tautomers have been calculated using anab initio Hartree-Fock-Roothaan SCF MO method. The calculated relative stabilities agree in general with the results of earlier semiempirical studies. According to the present study, tautomeric forms with regular Kekulé structure for the six-membered purine ring are the most stable. The amine-imine tautomerization of purine bases is not likely to be responsible for spontaneous mutations in 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.

Coupled-cluster studies. I. Application to small molecules, basis set dependences

Abstract The coupled-cluster doubles method for the calculation of the correlation energy in molecular systems is implemented in an ab initio framework. The use

Perturbational approach to the interaction between two nearly incommensurable polymers

The perturbational method developed in the present paper is applied to a few simple model systems, to two interacting hydrogen molecules–polymers and to polymeric systems of hydrogen molecules and hydrogen fluoride molecules. The validity of our method is studied by comparing the results obtained with the perturbation method and the tight‐binding SCF crystal orbital method. The obtained total electronic energies and the charge distributions are in good agreement with each other for both methods. This result leads to the conclusion that the present perturbational approach is promising for the application to the interactions between real, incommensurable polymers.

Peptide models VII The ending of the right-handed helices in oligopeptides [For-(Ala)n-NH2 for 2 ⩽ n ⩽ 4] and in proteins

Abstract The right-handed helical conformation (denoted as αL) of a single amino-acid diamide (e.g. HCONHCHCH3CO NH2) is not a minimum energy conformation on the ab initio potential energy surface. Computations performed on oligopeptides [For-(Ala)n NH2, for n ⩽ 4], revealed that the helix-like conformations do exist if the backbone conformation at the carboxyl-end is of δL type; i.e. (αL)n − 1δL. This suggests that according to SCF computations, the isolated helices end in a type I β-turn (αLδL).

Numerical application of the coupled cluster theory with localized orbitals to polymers II. Optimal localization of Wannier functions and the correlation energy in different approximations

Abstract We present an iterative method to obtain localized Wannier functions, needed in the framework of correlation energy calculations on polymers with different size-consistent methods using a localized orbital basis. Test calculations using different possible localization schemes are performed on alternating all-trans polyacetylene (t-PA), which is an example for polymers with covalently bound unit cells. The improvement of the localization is compared with respect to the total correlation energy per unit cell at the level of second order orbital invariant Moller-Plesset perturbation theory (LMP2) to the canonical MP2 (CMP2) method, and also results of the calculation of the correlation energy with the coupled cluster doubles theory (CCD) and its linear approximation (LCCD) are shown, We found that the coupled cluster expansions failed to converge for systems containing the Wannier functions belonging to two interacting unit cells if their interactions are too large (in case of a double zeta basis set and optimally localized Wannier functions). This is probably due to linear dependences in the systems of equations for such a highly symmetric system. Such a behavior can be made plausible with the help of a very simple model. Possibilities to overcome this problem are discussed. However, since in this work we are mainly concerned with the localization properties of Wannier functions in correlation calculations, we concentrate on comparisons of the correlation energy obtained with our localized orbital approximation to the energies as computed in the corresponding canonical orbital basis. Since the latter ones are available only for MP2 we concentrate in the present paper on this method, which can be viewed as a second order approximation to the coupled cluster expansion for double excitations. A comparison of the influence of the localization approximation on the correlation energy obtained with the corresponding canonical procedure is made for Clementis minimal and double zeta basis sets on the MP2 level and, in addition, the localized Wannier functions of larger systems and the effects of the localized orbital approximation on a potential curve for t-PA are discussed.