An investigation of an LCAO–MO–SCF method for co-ordination compounds based on Mulliken-type approximations

Abstract

We have investigated the applicability of a molecular orbital method for co-ordination compounds, based on Mulliken-type approximations, using TiF₆^3– as a control system. The method requires empirical evaluation of one-centre parts of the matrix elements and theoretical calculation of overlap, kinetic, Coulomb, and exchange two-centre integrals and of three-centre nuclear attraction integrals. Open-shell characteristics and electronic repulsion corrections for transition energies are taken into account. The use of several fixed configurations, or of a variable configuration, for the atomic eigenfunctions is tested. The calculations indicate a large participation of 4s and 4p orbitals to a very covalent bond, although a relatively small amount of π-bonding is present. However, the occupation numbers and some transition energies often become unacceptable. There appears to be evidence that this may be due to the lack of correlation for the two-centre interactions. In fact, all the variations that act in the same sense of a two-centre correlation tend to improve the results.