Approximate Inclusion of Triple Excitations in Combined Coupled Cluster/Molecular Mechanics: Calculations of Electronic Excitation Energies in Solution for Acrolein, Water, Formamide, and N-Methylacetamide.

Electronic excitation energies are often significantly affected by perturbing surroundings such as, for example, solvent molecules. Correspondingly, for an accurate comparison between theory and experiment, the inclusion of solvent effects in high-level theoretical predictions is important. Here, we introduce the CCSDR(3)/MM model designed for an effective, flexible, and accurate prediction of electronic excitation energies in solution. The method is based on a hybrid coupled cluster/molecular mechanics (CC/MM) strategy including interactions between a solute described by CC methods and a solvent described by polarizable MM methods. The CCSDR(3)/MM includes triples effects in a computational tractable noniterative fashion. The resulting approach allows for both high-accuracy inclusion of triples effects and inclusion of solute-solvent interactions with polarization effects, as well as being applicable for averaging over many solvent configurations derived from, for example, molecular simulations. We test the proposed model using as a benchmark the two lowest-lying valence singlet excitations (n → π* and π → π*) of acrolein, formamide, and N-methylacetamide in aqueous solution as well as liquid water, demonstrating how a systematic inclusion of many different effects leads to good agreement with experimental values. In doing so we also illustrate the theoretical challenges involved when investigating UV properties of solvated molecules.