THE GENERALIZED SYMMETRY ADAPTED CLUSTER THEORY

The generalized version of the symmetry adapted cluster theory is developed for general classes of nonclosed-shell cases. We started with the restricted Hartree-Fock complete active sapce self-consistent-field wave function but redefined a reference function which includes the state-specific correlation effects. The transferable dynamical correlation effects are expressed through an exponential operator and a linear operator is used to represent the nontransferable state-specific correlation effects such as quasidegeneracies, internal and semi-internal correlation effects, the spin-polarization effect, etc. The present theory is exact and does not involve any noncommutative algebra. There is a very close parallel between the standard single-reference cluster theory and the present multireference version. We have also discussed some aspects of practical applications of the generalized symmetry adapted cluster theory.