Solvent effects on optical properties of molecules: a combined time-dependent density functional theory/effective fragment potential approach.

A quantum mechanics/molecular mechanics (QM/MM) type of scheme is employed to calculate the solvent-induced shifts of molecular electronic excitations. The effective fragment potential (EFP) method was used for the classical potential. Since EFP has a density dependent functional form, in contrast with most other MM potentials, time-dependent density functional theory (TDDFT) has been modified to combine TDDFT with EFP. This new method is then used to perform a hybrid QM/MM molecular dynamics simulation to generate a simulated spectrum of the n-->pi(*) vertical excitation energy of acetone in vacuum and with 100 water molecules. The calculated water solvent effect on the vertical excitation energy exhibits a blueshift of the n-->pi(*) vertical excitation energy in acetone (Delta omega(1)=0.211 eV), which is in good agreement with the experimental blueshift.