Solvent effects on the 1(n, π∗) transition of formaldehyde in liquid water. A QM/MM study using the mean field approximation

In this Letter we propose a new method for the study of solvent effects on electron spectra that combines quantum mechanics (QM) in the description of the solute molecule and molecular mechanics (MM) in the description of the solvent. Unlike other QM/MM methods, the solvent perturbation is introduced into the solute molecular Hamiltonian in an averaged way, i.e., we use a mean field approximation. The method is implemented at the multiconfigurational self-consistent field and configuration interaction levels. Numerical results for the solvent shift of formaldehyde in liquid water are presented.