An ab initio model for solvent effects in organic molecules

An ab initio formulation for calculating solvent effects for organic molecu les is presented. The solvent effects are treated in two parts using differ ent models for the solvent. For calculating the reaction field the solvent is modeled as a continuum with the cavity determined ab initio as a surface enclosing the solute molecule, which represents the minima of the interact ion potential as a solvent molecule approaches a solute molecule at various angles. The interaction potential is calculated using results of ab initio diatomic calculations on various pairs of atoms with frozen asymptotic cha rge densities. The reaction field contribution from the solvent is evaluate d by using the apparent surface charge model with a dense grid of points on the cavity, For the direct interaction we first construct the first shell of solvent molecules around the solute molecule by bringing the solvent mol ecules as discrete systems to the cavity surface. The corresponding energy (consisting of both the solvent-solute and solvent-solvent interaction) is minimized with respect to both the location of solvent molecules as well as their orientation. The method is demonstrated by application to the p-nitr oaniline in various solvents. The solvated excitation energies are calculat ed and compared with experiment. We also compute the solvated polarizabilit ies and second-order transition moments.