A combined discrete/continuum solvation model: Application to glycine

A new solvation model that combines discrete and continuum descriptions of the solvent has been developed. The discrete solvent molecules are represented by effective fragment potentials (EFP), while the continuum is represented by the Onsager model. This (EFP+Onsager) model has been applied to the relative stabilities of the neutral and zwitterionic forms of glycine. Other supermolecule-continuum calculations were also performed, using quantum mechanical discrete waters and the isodensity polarizable continuum model (IPCM) or solvation model 5.42R (SM5.42R) for the continuum. It is shown that the Onsager model provides a poor description of the solvent in the supermolecule-continuum calculations. On the other hand, more sophisticated models can predict the correct energy order of the glycine isomers. Thus, the development of mixed methods that combine sophisticated continuum models with the discrete EFP model appear to be promising.