SOLVATION ENERGIES AND ELECTRONIC-SPECTRA IN POLAR, POLARIZABLE MEDIA- SIMULATION TESTS OF DIELECTRIC CONTINUUM THEORY

A dielectric continuum theory for the solvation of a polar molecule in a polar, polarizable solvent is tested using computer simulations of formaldehyde in water. Many classes of experiments, for example those which measure solvent-shifted vertical transition energies or electron transfer rates, require an explicit consideration of the solvent elec tronic polarization. Due to the computational cost of simulating a pol arizable solvent, many simulation models employ non-polarizable solute and solvent molecules and use dielectric continuum theory to relate t he properties of the non-polarizable system to the properties of a mor e realistic polarizable system. We have performed simulations of groun d and excited state formaldehyde in both polarizable and non-polarizab le water, and the solvation energies and solvent-shifted electronic sp ectra we obtained are used to test dielectric continuum, linear respon se predictions. Dielectric continuum theory correctly predicts that fr ee energy differences are the same in polarizable and non-polarizable water. The theory wrongly predicts that the reorganization energy in a polarizable solvent is 30% smaller than the reorganization energy in a polar, non-polarizable solvent; in the simulations, the reorganizati on energies differ by only 6%. We suggest that the dielectric continuu m theory fails because it assumes that both solute electronic states e xist in the same size cavity in the solvent, whereas in the simulation the cavity radius increases by 20% after the electronic transition. W e account for the change in the cavity size by adding a non-linear sol ute-solvent coupling to the dielectric continuum theory, and find that the resulting predictions are just outside the error bounds from the simulation. The cavity size corrections have the undesired and incorre ct side-effect of predicting fluctuations far smaller than seen in the simulations. This reveals the inherent difficulty in devising a simpl e, fully self-consistent dielectric continuum theory for solvation. (C ) 1996 American Institute of Physics.