APPLICABILITY OF SINGLE-REFERENCE COUPLED-CLUSTER METHODS TO EXCITED-STATES - A MODEL STUDY

It is demonstrated that standard single-reference coupled-cluster (SR- CC) theories can be successfully applied to the studies of some excite d states not belonging to the lowest energy in their symmetry class. T he prerequisite of such applications is that a proper reference state configuration and orbital set are employed for the parametrization of the SR-CC method. The 2 1A1 and 5 1A1 states of the simple H4 model sy stem are considered for geometries related to various degrees of quasi degeneracy of these states. It is found that the efficiency of various versions of the SR-CC method (except the linearized versions) is simi lar to that of standard applications to the closed-shell ground state of H4.