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On the Importance of the Orbital Relaxation in Ground-State Coupled Cluster Calculations in Solution with the Polarizable Continuum Model of Solvation.

In the description of the electrostatic interaction between a solute treated at coupled cluster (CC) level of theory and a solvent modeled as a continuum dielectric, the solvent response depends on various contributions: the choice of the reference wave function, the correlation density, and the orbital relaxation. In previous work, we examined the first two factors with the coupled cluster singles and doubles (CCSD) method and its variant Brueckner doubles (BD) method. The CC wave function was combined with the polarizable continuum model (PCM) of solvation in an integrated and efficient method able to describe energy and molecular properties through analytic energy gradients. Additionally, we investigated some approximations, and proposed new ones, that reduce the computational cost to nearly that of gas phase CC while keeping most of the complete model description. In this work, we study the contribution of the orbital relaxation and compare it to the other effects. Such contribution is introduced with a self-consistent macroiteration procedure, where the reaction field is updated with a refined density. The results presented here show that the effect of the orbital relaxation is small for CCSD, while for BD the integrated and self-consistent approaches are equivalent. Thus, these results further confirm that the integrated CCSD-PCM and BD-PCM methods, especially with their respective approximations, are an efficient approach to perform high-level electronic structure calculations in solution.