Abstract
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.