A general constrained geometry optimization procedure for charged molecules in an external electric field is developed. The procedure uses constraints to neutralize the resultant net force between the charged molecule and the external electric field. Three different constraints are explicitly considered : (1) a fixed center of coordinates, (2) a fixed center of mass, and (3) a fixed single atom. Field-dependent optimized geometries and vibrational frequencies for F-. H2O and I-. H2O are obtained and shown to be constraint-dependent. The constrained center of coordinates appears to simulate the charged system at static equilibrium in the electric field. The fixed atom constraint models the situation when an atom becomes attached to a more extended substrate. The center-of-mass constraint gives optimized structures that are mass sensitive and intermediate between those obtained with the other two constraints. The constrained geometry optimization procedure has applications to modeling different environments on electrode surfaces.