DIRAC-FOCK-ROOTHAAN CALCULATIONS USING A RELATIVISTIC REDUCED FROZEN-CORE APPROXIMATION

The reduced frozen-core approximation (RFCA) that has been previously used for nonrelativistic calculations is extended to relativistic calc ulations. The RFCA adopts a new method for the orthogonalization of va lence basis functions to core orbitals. Orthogonalization is performed using corelike basis functions consisting of fewer primitive basis fu nctions than core orbitals. Dirac-Fock-Roothaan calculations on HI and ThO show that the relativistic RFCA can reduce computing time and clo sely reproduce the total and valence orbital energies and spectroscopi c constants obtained by all-electron calculations. (C) 1998 American I nstitute of Physics. [S0021-9606(98)30443-2].