James E. Eilers

Utilization of Transferability in Molecular Orbital Theory

Publisher Summary This chapter discusses the utilization of transferability in molecular orbital theory. The chapter elaborates on the ways in which transferability can be utilized in the construction of wavefunctions for large systems. The chapter explores those methods that use such transferability to mimic ab initio calculations on large molecules, and the role played by transference in improving the numerical convergence of self-consistent field (SCF) cycling procedures. Within the molecular orbital theory, the transference of matrix elements from one molecule to another and their possible use in the construction of wavefunctions for large systems forms a natural alternative to the transferability of the parts of a wavefunction, such as localized orbitals. The chapter discusses various methods involving the transfer of matrix elements that includes the Non EmpiricaI Molecular Orbital (NEMO) scheme, and Simulated Ab initio Molecular Orbital (SAMO) scheme. The chapter presents the transference methods involving the use of localized orbitals, and finally considers the use of both transferred matrix elements and transferred localized orbitals in improving the efficiency of SCF cycling procedures.

The simulated ab initio molecular orbital method for polymers polyglycine

Abstract The simulated ab initio molecular orbital (SAMO) method is here applied to polyglycine — the simplest polymer of biochemical interest. Results are compared with semi-empirical methods and experiment.

A modified hamiltonian method for the study of multiple site reactivity: comparison with perturbation results

Abstract The interaction field modified hamiltonian (IFMH) method is presented, in which the polarization of the electron density in a molecule A by the electrostatic field generated by the electron density distribution and the model of a molecule B is calculated by a variational procedure. The method is designed to be equivalent to an infinite expansion of the polarization interaction between the molecules, as described previously by a multiple perturbation method. Results from the two approaches are compared for their ability to predict the changes induced in the reactivity of the attacked molecule A by the reagent B. The effects of truncation of the perturbation expansion and of the partial antisymmetizatrization approximation that is common to both IFMH and the perturbation method, are discussed in comparison with a full supermolecule calculation of the AB complex. The numerical comparisons are for the interaction of formyl fluoride (HCOF) with HF at hydrogen-bond distances, calculated ab initio with a contracted (5s, 3p, 2s/2s, p, s) gaussian basis.

Theoretical studies of molecular complexes: a probe into basis set and correlation effects

Abstract We investigate the effect of basis set size, correlation effects and interplanar separation on the theoretical electronic structure of stacking complexes of para- and meta-hydroxyaruline with formamidinium cation, constructed as analogs for the complexes of 5- and 6-hydroxytryptamine with imidazolium cation.

Simulated ab initio molecular orbital technique. Part 5.—Polar groups, ionic molecules and orthogonalised basis sets

The previously reported simulated ab initio molecular orbital (SAMO) method is extended in several ways. First, the application to polar molecules is demonstrated for the first time. The systems studied are RX, where X = OH, NH2, CHO or COOH and R is an alkyl group. Results are at least as good as for the non-polar molecules previously studied. Secondly, a simple extension is developed for RX molecules where X is ionic. For these systems the simple method is inapplicable. Finally, the use of an orthogonalised basis is described and found to be less successful than expected.