An ab initio study, using MP2/6-31G(d,p) and MP2/cc-pVDZ levels of theory, of the pentamethylene sulfide...HCl potential energy surface gives two chair-like complexes involving axial and equatorial hydrogen bonds as the most stable structures. Their computed geometrical parameters and rotational constants are in good agreement with the experimental ones. The basis set superposition error is crucial in giving the equatorial conformer as the most stable structure in accord with the experimental observation. Both the high barrier to interconversion between the axial and equatorial conformers and their low energy difference explain why the intensity ratio experimentally observed for them remains unchanged when different carrier gases are employed.