We report the results : of a comprehensive computational investigation on the energetics of sulfur mustard (S(CH2CH2Cl)(2)) conformations. Molecular mechanics (MM2*, MM3*, AMBER*, and OPLS*), semiempirical (MNDO, AM1, and PM3), and ab initio (HF/3-21G, HF/6-311G**, MP2/6-31G*, and MP2/6-311G**/ /HF/6-311G**) methods were applied to 12 low-energy structures obtained from a Monte Carlo conformational search using force fields contained in MacroModel 3.5a. In general, there is reasonable agreement between molecular mechanics and ab initio for geometrical properties, but significant differences in the energy predictions. There was more scatter from the semiempirical computations, with the AM 1 model most successfully reproducing the ab initio results. The ab initio calculations identify at least three conformations of sulfur mustard lower in energy than the all-anti structure, depending on the level of theory employed. Vibrational infrared spectra were computed for the four lowest energy structures at the HF/6-311G** level and compared to experimental data, producing a better match than obtained from consideration of a single conformer, but at greater computational cost.