An ab initio model for solvent effects in organic molecules

An ab initio formulation for calculating solvent effects for organic molecules is presented. The solvent effects are treated in two parts using different 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 interaction 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 charge densities. The reaction field contribution from the solvent is evaluated 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 molecules 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-nitroaniline in various solvents. The solvated excitation energies are calculated and compared with experiment. We also compute the solvated polarizabilities and second-order transition moments.