Polarizable embedding cluster perturbation theory for a coupled cluster singles and doubles target state

This study presents the polarizable embedding cluster perturbation framework (PE-CP) and derives the working equations needed to perform calculations in a singles and perturbative doubles excitation space, denoted PE-CPS(D). The method is implemented in the Penguin program through interfaces to the external libraries PyFraMe and CPPE. To obtain the cluster amplitudes and multipliers, which constitute two coupled sets of equations in PE-CCSD, a standard scheme of outer and inner iterations is usually applied. In the PE-CPS(D) model, only the singles excitation space requires this iterative procedure to determine the wave function parameters; the perturbative doubles contributions can be evaluated directly. This feature highlights one of the principal strengths of the PE-CPS(D) approach. Finally, we present numerical tests comparing PE-CPS(D) with PE-CCSD, to which it formally converges. These results demonstrate that PE-CPS(D) provides an efficient route to high-level coupled-cluster accuracy for large quantum-mechanical systems embedded in a classical polarizable environment.