An Ab Initio QM/MM-Based Approach to Efficiently Evaluate Vertical Excitation Energies in Condensed Phases Including the Nonequilibrium Solvation Effect

An efficient quantum mechanical/molecular mechanics-based approach is presented to calculate a vertical excitation energy of a chromophore in condensed phases including the nonequilibrium solvation effect. The electronic polarization of a medium and the related nonequilibrium solvation effect associated with the vertical excitation are described using a polarizable solvent model. By virtue of the mean-field approximation, the target energy can be completely separated into classical and quantum mechanical parts, which enables us to efficiently evaluate the vertical excitation energy with a high-level quantum mechanical method. The method is applied to N,N-dimethyl-4-nitroaniline in a variety of solutions at the MRMP2/CASSCF level, showing quantitative agreement with the experimental reports. The observed large bathochromic shifts are analyzed by focusing on the induction effects of the solvents.