Nonequilibrium molecular dynamics simulations of water have been performed in the isothermal-isobaric ensemble in the presence of external electromagnetic fields of varying intensity in the microwave to far-infrared frequency range, using a rigid/polarizable and a flexible/nonpolarizable potential model, from 260 to 400 K. Significant alterations in molecular mobility and hydrogen bonding patterns were found vis-a-vis zero-field conditions. In addition, the influence of the isothermal-isobaric ensemble on these observations was gauged by means of comparison with pure Newtonian simulation findings in the presence of external fields, and the former results were in reasonable accord with the latter. (C) 2003 American Institute of Physics.