A 2-STEP METHOD OF CALCULATION OF THE ELECTRONIC-STRUCTURE OF MOLECULES WITH HEAVY-ATOMS - THEORETICAL ASPECT

An approach for a space-separated calculation of the wave function in the valence and core regions of a molecule is proposed. As the first s tep, the calculation of the orbitals (or two-component spinors in the relativistic case) in the valence region by the effective core potenti al (ECP) method is performed. Then, it is followed by a restoration of orbitals (four-component spinors) expanded on spherical harmonics in the core regions of heavy atoms. Theoretical questions of the variatio nal calculation of the molecular orbitals are considered in some core region Limited by a sphere. Inclusion from the electronic cloud outsid e this region is reduced by the necessity of taking into account the o rthonormality and boundary conditions together with an effective exter nal field in respect to the selected core region. This method may be u sed for calculation of matrix elements of operators that are singular near nuclei (P,T-odd interactions, hyperfine structure, etc.). A subst antial computational saving can be reached because the method enables, by the most optimal way, to combine the advantages of two well-develo ped approaches: molecular ECP calculations in the Gaussian basis set a nd one-center numerical atomic calculations with an external field. It is especially important when the relativistic effects are taken into account. (C) 1996 John Wiley & Sons, Inc.