The accuracy of expanding the response of a molecule to an external electric field, E, as a power series in the field is investigated in the model hydrogen-bonded complex, ClH:NH3. Even at field strengths large enough to cause dramatic structural change in the complex, both the structure and vibrational frequencies are quantitatively predicted using only terms linear in E. These results suggest that knowledge of the zero-field molecular potential energy and dipole moment surfaces may be sufficient to accurately model the interactions of molecules in a wide range of external electric fields. (C) 2003 Elsevier Science B.V. All rights reserved.