Generalized relativistic effective core potential: Theoretical grounds

The generalized relativistic effective core potential (GRECP) method is ana lyzed from theoretical and computational points of view. The Hamiltonian in the frozen-core approximation is compared with the Hamiltonian containing the GRECP operator. It is demonstrated that the GRECP operator can be deriv ed from rather natural physical grounds and the procedure of the GRECP gene ration can be justified theoretically. The accuracy of the RECP approximati ons in the simulation of the interactions and densities in the valence acid outer-core regions is analyzed. The reliability of the simulation of the i nteraction with the inner-tore electrons removed from the calculations with the GRECP is also studied. The importance of additional nonlocal terms bot h with the potentials for the outer-core pseudospinors and with the potenti als depending on the occupation numbers of the outermost core shells in the expression for the GRECP operator is demonstrated in calculations on the A g, Ba, Hg, Tl, and U atoms. The difference between the outer core and valen ce potentials was investigated. It is shown that in the valence region the two-component pseudospinors coincide with the large components of four-comp onent spinors in calculations for the same configuration states with a very high accuracy. Problems of Gaussian approximation caused by rather singula r shapes of the potentials are considered. To attain a required high accura cy of approximation of the numerical potentials by Gaussians, serious addit ional efforts were undertaken. (C) 1999 John Wiley & Sons, Inc.