Generalization of coupled-cluster response theory to multireference expansion spaces: application of the coupled-cluster singles and doubles effective Hamiltonian

Employing Separate cluster ansatz in time-independent and time-dependent wa ve-operators, coupled-cluster (CC) response theory is generalized to multir eference (MR) expansion spaces. For state energies, this corresponds to the MR secular problem with an arbitrary similarity-transformed effective Hami ltonian, (H) over tilde = Omega(-1) H Omega. The effective Hamiltonian can be generated via size-extensive CC methods. Thus the states in MR linear re sponse theory (MRLRT) maintain the usual CC core-extensive properties. We h ave used the Gelfand unitary group basis of the spin-adapted configurations to construct the matrix off? in the MR excitation space. As a preliminary application, the CC singles and doubles effective Hamiltonian is applied to excitation and photoionization energies of the CH+ and N-2 molecules, and is compared with experimental results and results from other numerical proc edures including conventional CC linear response theory (CC-LRT), MR and fu ll configuration interaction (MRCI and FCI) methods. The numerical results indicate that MRLRT reproduces valence and external excited states quantita tively, combining the best features of CC-LRT and MRCI.