Adding the linked contributions of Triples and Quadruples to a size-consistent Singles and Doubles CI

Abstract

A convenient procedure has been proposed recently to insure the size-consistence of Singles and Doubles CI; the method goes through a self-consistent dressing of the energies of the excited determinants which incorporates the unlinked effects of the Triples and Quadruples. Two strategies are proposed here to add thelinked contributions of the Triples and Quadruples, either by an perturbative MP-type calculation of these 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 methods efficiently approach the Full CI results (error ∼ 1 kcal mol−1). The second one satisfactorily treats the single-bonds breaking. It is finally shown that the effect of the T-Q linked effects may be efficiently approximated by truncating the MO basis set to the most occupied virtual quasi-natural MOs.