Development and pilot molecular applications of the uncoupled state-specific MRCC (UC-SS-MRCC) theory

In this paper, we propose and apply an uncoupled approximation to the rigorous, size-extensive state-specific multireference coupled cluster theory (SS-MRCC). developed earlier by Mukherjee and coworkers [U.S. Mahapatra, B. Datta, D. Mukherjee, J. Chem. Phys. 110 (1999) 6171]. Both the parent formulation and the uncoupled approximant use the Jeziorski-Monkhorst Ansatz: Omega = Sigma(mu) exp(T-mu)vertical bar phi(mu)> <,)(phi(mu)vertical bar involving a different cluster operator exp(T-mu) acting on its corresponding model function phi(mu). The approximant presented in this paper builds on a preliminary formulation presented by us recently [THEOCHEM 79 (2006) 771]. The working equations of the SS-MRCC, following a set of physically motivated sufficiency conditions, are characterized by certain projection amplitudes connecting each phi(mu) to each virtual function z1 which is equated to zero. These equations have two types of terms: a 'direct' term where for each phi(mu) only powers of T-mu appear, and a 'coupling' term where T-nu (nu not equal mu) also figures. In the uncoupled SS-MRCC approximant (UC-SS-MRCC) we explore the possibility of avoiding the coupling term by approximating the coupling matrix element by , where T-mu (nu)s form the subset of the components of T-mu operators which has the same excitation structure of those operators T-nu that give nontrivial contributions by their actions on phi(mu). Absence of the coupling terms leads to considerable simplifications of the working equations. Pilot molecular applications of the UC-SS-MRCC theory to the singlet states of CH2 and SiH2, which possess strongly multi-reference character and to the potential energy surface of the ground state of Li-2 indicate the extreme closeness of the computed energies with those from the rigorous but more involved parent SS-MRCC theory. This indicates the potentiality and the efficacy of the UC-SS-MRCC approximation. (c) 2008 Elsevier B.V. All rights reserved.