Ab initio calculations at the levels of Hartree-Fock (HF), second order Moller-Plesset perturbation theory (MP2), coupled-cluster theory with singles, doubles, and estimated triple excitations [CCSD(T)] levels, and density functional theory of the relative energies of C₂H₂S₂ isomers 1,2-dithiete (2a) and dithioglyoxal (2b) are performed. An unusual large dependence on the presence of f-type polarization functions in the basic sets are found on the ab initio calculations. The large change in the relative energies calculated at MP2 and CCSD(T) are found to be mainly due to the functions at sulfur.