MULTIREFERENCE PERTURBATION CI .1. EXTRAPOLATION PROCEDURES WITH CAS OR SELECTED ZERO-ORDER SPACES

We discuss the ''three class'' approximation to full multireference pe rturbation CI, which greatly reduces the computational effort by restr icting the summation of diagrams to determinants belonging to a subspa ce of the zero-order space. In the framework of the CIPSI algorithm, w e propose a new extrapolation procedure allowing recovery of the full ''two class'' results. The new procedure is applied to complete active spaces (CAS) and to individually selected zero-order spaces. Comparis on with a full two class calculation on a CAS shows a reduction of com puter time of one or two orders of magnitude in the tests presented he re, with an accuracy in the order of 0.1 kcal/mol. Our procedure can t hus compete with the CASPT2 algorithm, specifically conceived to deal with CAS. In the case of selected zero-order spaces, the speed-up is l ess dramatic but the method still retains its advantages.