A CSF-BASED MULTIREFERENCE COUPLED PAIR APPROXIMATION .2. AS A SIZE CONSISTENT METHOD IN THE SCHEME OF MULTIREFERENCE SINGLY AND DOUBLY-EXCITED CONFIGURATION-INTERACTION

MRCPA (multireference coupled pair approximation) is formulated by the use of the wave operator formalism and Rayleigh-Schrodiner perturbation theory with special selection of the unperturbed part of the electronic Hamiltonian. By considering super molecule, it is shown that the theory is size consistent with the help of the new formalism. The method has been tested for three simple systems, H2O, FH, and O2.