A size-consistent state-specific multireference coupled cluster theory: Formal developments and molecular applications

In this paper we present a comprehensive account of a manifestly size-consistent coupled cluster formalism for a specific state, which is based on a reference function composed of determinants spanning a complete active space (CAS). The method treats all the reference determinants on the same footing and is hence expected to provide uniform description over a wide range of molecular geometry. The combining coefficients are determined by diagonalizing an effective operator in the CAS and are thus completely flexible, not constrained to preassigned values. A separate exponential-type excitation operator is invoked to induce excitations to all the virtual functions from each reference determinant. The linear dependence inherent in this choice of cluster operators is eliminated by invoking suitable sufficiency conditions, which in a transparent manner leads to manifest size extensivity. The use of a CAS also guarantees size consistency. We also discuss the relation of our method with the extant state-specific formalisms. Illustrative applications are presented for systems such as H(4) in rectangular and trapezoidal geometries, the Be-H(2) C(2)v insertion reaction path, the potential energy surface of Li(2) and F(2), and certain states of CH(2) and C(2) molecules with pronounced multireference character. The results indicate the efficacy of the method for obviating the intruders and of providing accuracy. (C) 1999 American Institute of Physics. [S0021-9606(99)30612-7].