2ND-ORDER PERTURBATION ON A SDCI CALCULATION

Starting from a SDCI calculation, the SD eigenvector is perturbed by a ll triply and quadruply excited determinants. Efficiency is promoted t hrough direct CI techniques, however, some flexibility has been kept, making possible the use of various perturbational schemes (here: Epste in-Nesbet and Moller-Plesset). The SDCI starting point avoids divergen ce problems posed by CCSD(T) and also by purely perturbative methods s uch as MP4. Several calculations on the potential curves of some molec ules (H2O, N2, F2, Ne2) show that the present method is at least as go od as the MP4 or CCSD(T) methods at comparable computational cost.