Coupled cluster theory in the condensed phase within the singles-T density scheme for the environment response

Reliable simulations of molecules in condensed phase require the combination of an accurate quantum mechanical method for the core region, and a realistic model to describe the interaction with the environment. Additionally, this combination should not significantly increase the computational cost of the calculation compared to the corresponding in vacuo case. In this review, we describe the combination of methods based on coupled cluster (CC) theory with polarizable classical models for the environment. We use the polarizable continuum model (PCM) of solvation to discuss the equations, but we also show how the same theoretical framework can be extended to polarizable force fields. The theory is developed within the perturbation theory energy and singles-T density (PTES) scheme, where the environmental response is computed with the CC single excitation amplitudes as an approximation for the full one-particle reduced density. The CC-PTES combination provides the best compromise between accuracy and computational effort for CC calculations in condensed phase, because it includes the response of the environment to the correlation density at the same computational cost of in vacuo CC. We discuss a number of numerical applications for ground and excited state properties, based on the implementation of CC-PTES with single and double excitations (CCSD-PTES), which show the reliability and computational efficiency of the method in reproducing experimental or full-CC data. This article is characterized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Electronic Structure Theory > Combined QM/MM Methods Software > Quantum Chemistry