Multireference perturbation configuration interaction methods for solvated systems described within the polarizable continuum model

We present a method to include solvent effects described within the polarizable continuum model into the CIPSI multireference perturbation algorithm. In the methodology we have formulated and implemented, solvent interactions are explicitly included in the configuration interaction scheme and in the following perturbative corrections, through proper operators. The nonlinear character induced by such operators leads to an iterative procedure in which solute and solvent can mutually equilibrate. Applications to the electronic excitation spectrum of formamide are considered. In these cases, effects due to an incomplete electrostatic response of the solvent (nonequilibrium model) as well as repulsion interactions between solute and solvent have been included in the quantum mechanical description. (C) 1999 American Institute of Physics. [S0021-9606(99)30540-7].