Abstract Ab initio calculations have been performed on methane sulfonamide anion. Geometries have been optimized using Hartree-Fock basis sets up to 6-31+G*, and single-point calculations employing those Hartree-Fock geometries have been performed at levels up to MP2/6-311++G**. In addition, geometry optimizations for the 0, 90, 150, and 180 conformers have been carried out at the MP2/6-31G*, MP2/6-31+G*, and MP2/6-311++G** levels. Vibrational frequencies have been calculated using the HF/4-31G*, MP2/6-31G*, and MP2/6-31+G* geometries. All calculations at or above the 4-31G* level agree that HNSC 90 is the global minimum. The HNSC=180 conformer is clearly higher in energy although the relative energy of this conformer varies from 0.36 to 1.03 kcal/mol for the post-HF calculations depending on basis set. The HNSC =180 conformer appears to be a very shallow local minimum. However, the potential energy surface is quite flat in this region, and the highest-level calculations, including MP2 optimizations and vibrational frequency analysis, are ambiguous on this point. The conformer with an HNSC torsion of 0 is a transition state with a relative energy 8 kcal/mol.