ADDING THE LINKED CONTRIBUTIONS OF TRIPLES AND QUADRUPLES TO A SIZE-CONSISTENT SINGLES AND DOUBLES CL

A convenient procedure has been proposed recently to insure the size-c onsistence of Singles and Doubles CI; the method goes through a self-c onsistent dressing of the energies of the excited determinants which i ncorporates the unlinked effects of the Triples and Quadruples. Two st rategies are proposed here to add the linked contributions of the Trip les and Quadruples, either by an perturbative MP-type calculation of t hese effects, or by a redefinition of the dressing of the SD-CI matrix . Test calculations on Be2, FH, NH3 and F2 molecules show that both me thods efficiently approach the Full CI results (error approximately 1 kcal mol-1). The second one satisfactorily treats the single-bonds bre aking. It is finally shown that the effect of the T-Q linked effects m ay be efficiently approximated by truncating the MO basis set to the m ost occupied virtual quasi-natural MOs.