Ab initio self-consistent field Hartree-Fock (HF) calculations of the bis-(trifluoromethanesulphone) imide ion (TFSI), N(CF3SO2)(2)(-), using the 6-31G* Gaussian basis set, have been performed in order to investigate the potential energy surface (PES) through rotations about the S-N bonds and to obtain the minimum energy structure. Two different minimum energy geometries were obtained : one with C-1 symmetry and the other with C-2 symmetry, the latter with the lower energy is probably the global minimum geometry. The energy difference is small : 2.3 kJ mol(-1), and the energy barrier between the structures according to the PES only 3.9 kJ mol(-1). This results in a large internal flexibility of the anion by rotations about the two S-N bonds. The importance of the S-N-S angle for this property has also been investigated. The anion has technological importance as counter-ion to lithium and plasticizer in blends of Lisalts and poly(ethylene oxide), creating polymer electrolytes. The present study implies that the plasticizing effect of the ion in these systems is due to the large mechanical flexibility.