Solvation of the excited states of chromophores in polarizable environment: orbital relaxation versus polarization.

A hybrid quantum mechanics/molecular mechanics (QM/MM) method for the electronic excited states has been developed. The equation-of-motion coupled cluster with single and double excitations method (EOM-CCSD) is used for the QM region, while the effective fragment potential (EFP) method describes a MM part. The EFP method overcomes the most significant limitation of QM/MM by replacing empirical MM interactions and QM/MM coupling by parameter-free first-principles-based ones, while retaining the computational efficiency of QM/MM. The developed QM/MM scheme involves quantum-mechanical coupling of the electrostatic and polarization terms in the QM/MM Hamiltonian and allows accurate calculation of the electronic excited states of chromophores in various environments. Applications to the water complexes of formaldehyde and p-nitroaniline show that the orbital relaxation of the solute in the electric field of the solvent provides the majority of the solvatochromic effect, and the response of the polarizable environment to the density of the specific electronic state is much smaller in magnitude.