A modified ansatz for explicitly correlated coupled-cluster wave functions that is suitable for response theory

A modified ansatz for explicitly correlated coupled-cluster wave functions with a single correlation factor is set forward. It is based on the fixed amplitude ansatz of Ten-no [Chem. Phys. Lett. 398, 56 (2004)] to which an extra term is added that allows for the explicitly correlated description of singly excited configurations. The new approach has been implemented for coupled-cluster singles and doubles with the aid of automated techniques. Numerical results are presented for vertical excitation energies, and ground and excited state equilibrium distances and harmonic frequencies of diatomics. The new approach is shown to provide a nearly unbiased description of ground and predominantly singly exited states, and the improvements seen for explicitly correlated treatments of ground states, as compared to conventional orbital expansions, carry over to excited states. In addition, a correction for contributions from one-electron terms-which are not improved by the correlation factor-is suggested, again with focus on applicability to a response formalism.