HIERARCHY OF COUNTERPOISE CORRECTIONS FOR N-BODY CLUSTERS - GENERALIZATION OF THE BOYS-BERNARDI SCHEME

The counterpoise correction method of Boys and Bernardi is generalized for a consistent treatment of the basis set superposition error (BSSE ) of three-body and higher many-body effects along the line used by Wh ite and Davidson for the three-body terms in an ice model. The BSSE-co rrected energy terms are calculated recursively by using the hierarchy of one-, two-, three-body, etc. energy terms, and by recomputing each such contribution in basis sets of subclusters of increasing size. Th e scheme is illustrated by calculations on the He trimer at the RHF, M P2 and MP4 levels. The results suggest that properly BSSE-corrected cl uster calculations may combine refined two-body interactions with appr oximate many-body corrections. The method might be useful in calculati ng potential surfaces of chemical reactions. (C) 1997 Published by Els evier Science B.V.