Glutamic acid in the gas phase and in aqueous solution was studied with ab initio quantum chemistry calculations. The effective fragment potential (EFP) has been used to study the effects of up to ten water solvent molecules on the neutral and zwitterionic isomers of glutamic acid, as well as to model the reaction path connecting these two isomers. The results with one and two water molecules have been compared to full ab initio ''super molecule'' calculations. This comparison shows that the EFP successfully models the effects of solvent molecules on structure and energetics. In the calculations including zero, one, or two water molecules, the neutral isomer was shown to be more stable than the zwitterion. On the other hand, in the EFP calculations with ten water molecules, where Monte Carlo methods were used to assist in finding the lowest energy conformations, the zwitterion was found to be more stable, which is the expected aqueous phase results. (C) 1997 American Institute of Physics.