Brueckner-type reference determinants in applications of coupled cluster methods to excited states

Model studies have been undertaken of the efficiency of single-reference state coupled cluster (SR-CC) methods defined in terms of several Brueckner type orbitals in applications to excited states of a predominately closed shell structure. The H4 model was used, which offers an easy way of representing any set of symmetry adapted orbitals together with the possibility of varying the degree of quasi-degeneracy of the states considered. Standard Brueckner orbitals (BOs) and orbitals generated in the projective and variational Brueckner CC (BCC) methods have been obtained and compared for the first time for excited states. Some attention has been paid to the stability of the BOs. Results of applying Brueckner-type and RHF orbitals in CCD, CCSD, and CCSDT calculations for excited states of rather strong degrees of quasi-degeneracy are presented. It is shown that in most cases proceeding from RHF orbitals to Brueckner-type orbitals results in an improvement in the accuracy of the SR-CC energies. Differences and similarities in the characteristics of Brueckner-type orbitals and SR-CC results for excited states and the ground state are identified and discussed.