6TH-ORDER MANY-BODY PERTURBATION-THEORY .1. BASIC THEORY AND DERIVATION OF THE ENERGY FORMULA

The general expression for the sixth-order Moller-Plesset (MP6) energy , E(MP6), has been dissected in the principal part A and the renormali zation part R. Since R contains unlinked diagram contributions, which are canceled by corresponding terms of the principal part A, E(MP6) ha s been derived solely from the linked diagram terms of the principal p art A. These have been identified by a simple procedure that starts by separating A into connected and disconnected cluster operator diagram s and adding terms associated with the former fully to the correlation energy. After closing all open disconnected cluster operator diagrams , one can again distinguish between connected and disconnected energy diagrams, of which only the former lead to linked diagram representati ons and, therefore, contributions to E(MP6). The connected diagram par ts of A have been collected in four energy terms E(MP6)(1), E(MP6)(2), E(MP6)(3), and E(MP6)(4). The sum of these terms has led to an approp riate energy formula for E(MP6) in terms of first- and second-order cl uster operators. (C) 1996 John Wiley & Sons, Inc.