A fast ab initio model for the calculation of excited electronic states ofatoms and molecules in a weakly polarizable environment. I. Theory

We report the development of an ab initio scheme designed for the calculati on of the electronic ground state and low-lying excited states of an atom o r a molecule, perturbed by a weakly interacting environment of discrete, un polarizable particles acting as a solvent. The model employs an ab initio p artitioning ansatz that accounts for the effects of nonlocal exchange-overl ap interactions between the solute and the solvent by means of a parametriz ed exchange-overlap operator and an effective metric in the pair-permutatio n, pair-additivity approximation, which is known to be valid in regions of small intermolecular overlap. Intramolecular perturbations like spin-orbit effects can be incorporated in the treatment. Due to its fast performance a nd built-in size-consistency, our model can be employed in the calculation of the electronic states of spectroscopically active fragments with many di fferent settings of the environment. (C) 2000 American Institute of Physics . [S0021-9606(00)30701-2].