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

We present a method to include solvent effects described within the polariz able 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 nonlinea r character induced by such operators leads to an iterative procedure in wh ich solute and solvent can mutually equilibrate. Applications to the electr onic excitation spectrum of formamide are considered. In these cases, effec ts due to an incomplete electrostatic response of the solvent (nonequilibri um model) as well as repulsion interactions between solute and solvent have been included in the quantum mechanical description. (C) 1999 American Ins titute of Physics. [S0021-9606(99)30540-7].