Multiconfiguration coupled-cluster calculations for excited electronic states: Applications to model systems

Abstract

Abstract A multiconfigurational coupled-cluster method previously developed in our laboratory is used to study excited states of the same spatial and spin symmetry as the ground state. Applications are made, with rather small atomic orbital basis sets, to molecular systems which are highly correlated. These small-basis calculations are viewed on model calculations whose value lies in the fact that one can also obtain the exact (full configuration interaction) energy in such cases. The results show that even though the coupled-cluster equations may have many spurious solutions, one can locate solutions corresponding to the desired excited states by using procedures similar to those utilized for ground states. To achieve this success, one should include in the reference function all of the dominant configurations of the state under consideration. Next, one should use the unique solution of the linearized coupled-cluster equations as the initial estimate to begin the solution of the non-linear coupled-cluster equations. If the solution of these non-linear equations gives rise to one or more large t amplitudes one should repeat this procedure but with the configuration corresponding to the large t amplitude included in the reference function.