An efficient computational scheme for using the relativistic and
non-relativistic quasidegenerate perturbation theory with general
multiconfigurational reference functions is implemented. The scheme is
based on the matrix element between the reference and the zero- to
two-electron ionized determinants. Comparison with a previous scheme
based on diagrams is made using the excitation spectra of
[PtCl4]2− , CH3I, and H2CO
as examples, and the efficiency of the scheme is illustrated.