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

摘要

A modified ansatz for explicitly correlated coupled-cluster wave functions with a single correlationfactor 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 singlyexcited configurations. The new approach has been implemented for coupled-cluster singles anddoubles with the aid of automated techniques. Numerical results are presented for vertical excitationenergies, 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 predominantlysingly 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, acorrection for contributions from one-electron terms—which are not improved by the correlationfactor—is suggested, again with focus on applicability to a response formalism.