Geometry optimization of molecules in solution: Joint use of the mean field approximation and the free-energy gradient method

The average solvent electrostatic potential/molecular dynamics (ASEP/MD) and the free-energy gradient methods are applied together with the multidimensional geometry optimization of molecules in solution. The systems studied were formamide in aqueous solution and water and methanol in liquid phase. The solute molecules were described through ab initio quantum mechanics methods (density dunctional theory or Møller–Plesset second order perturbation theory) while the solvent structure was obtained from Molecular Dynamics calculations. The method is very efficient; the increase in computation time is minimal with respect to previous ASEP/MD versions that worked at a fixed geometry. Despite the use of the mean field approximation in the calculation of the solvent reaction potential the agreement with previous theoretical calculations was satisfactory. Large changes were observed in the solute charge distribution induced by the solvent, and the solute polarization was accompanied by an increase in the solvent structure around the solute.