HARTREE-FOCK SOFT COULOMB HOLE TO ESTIMATE CORRELATION ENERGIES IN ATOMS, MOLECULES AND POLYMERS

We have shown that the empirical correction introduced into the Hartre e-Fock method to calculate correlation energies for atoms and therefor e to remove the error caused by the so-called Coulomb hole can be exte nded from atoms to molecules and polymers. A reformulation was require d of the necessary parameter representation. The reparametrization has been performed staying as close as possible to the original expressio ns for atoms reported by Chakravorty and Clementi (S.J. Chakravorty an d E. Clementi, Phys. Rev. A, 39 (1989) 2290). In addition to their wor k, where the correlation energy has been calculated with the self-cons istent Hartree-Fock wavefunction and the correction integrals, we have performed investigations, including the perturbation operator in the Fock operator, so that the total energy also contains the correlation energy. The applications of this approach to atoms and molecules show that the total electron correlation energies and ionization potentials calculated as differences of total energies can be obtained very sati sfactorily. On the basis of the reported calculations it turns out tha t one obtains better agreement with reference values of more sophistic ated calculations when the correction integrals are used to build up t he Fock matrix. Furthermore we have found that the magnitude of the co rrelation energy depends only weakly on the size of the basis sets, wh ich makes this empirical method very attractive for its application to large molecular and polymeric systems.