Description of intraatomic correlations by the Local Approach

Abstract

The Local Approach for calculating correlation energies of molecules is formulated for intraatomic correlations. This is an extension of earlier work in which it was successfully applied to interatomic correlations. In order to treat intraatomic correlations the atomic volume is divided into subspaces. The Hartree-Fock ground state is modified by decreasing electronic double occupancies of those subspaces. This leads to a generation of a correlation hole. A detailed description is given for the construction of the required subspaces. The theory is applied for the purpose of demonstration to the He and Be atoms, to the H₂-molecule and to a system of two He atoms. A comparison with CI-calculations shows that only few percents of the correlation energy are missing when similar basis sets are used. The advantage of the Local Approach is its intrinsic simplicity and that it operates in a much smaller configuration space than the CI-method. This makes it an appropriate method for the treatment of larger systems.