The behavior of the electronic structure in a metal/molecular/metal junction as a function of the applied electric field is studied using density functional methods. Although the calculations reported here do not include the electrode bulk, or intermolecular interactions, and do not permit actual transport to occur, nevertheless they illuminate the charging, energy shift, polarization and orbital occupation changes in the molecular junction upon the application of a static electric field. Specifically, external electric fields generally induce polarization localization on the two cluster ends. The HOMO/LUMO gap usually decreases and, for large enough fields, energy levels can cross, which presages a change of electronic state and, if found in molecular electronic circuits, a change in transmission. The calculations also show changes in the geometry both of the molecule and the molecule/cluster interface upon application of the electric field. These effects should be anticipated in whole circuit studies. (C) 2004 American Institute of Physics.