Convergency studies of second and approximate second order multiconfigurational Hartree−Fock procedures

The convergency of second order and approximate second order multiconfigurational Hartree−Fock procedures has been examined. Preliminary calculations on Be and O2 show that second order procedures straightforwardly can be applied and converged in a few iterations both for ground and excited states. Approximate second order schemes such as the ’’super CI’’ approach converged much slower. The present calculations preserve orbital orthogonality explicitly in each iteration without using a Schmidt, Löwdin, or other orthogonalization procedure.