Hartree-Fock, post-Hartree-Fock (MP2 and CCSD(T)) and density functional theory (BLY-P, B3LYP, BPW91 and B3PW91) methods were employed to obtain the relative stabilities of eight energetically competitive C2S2H2 isomers. Peculiarly high dependency on the basis set has been observed and the convergence of the relative stability orderings is slow even at the quintuple-zeta levels. Basis sets starting from 6-31G*, 6-311G*, 6-311+G*, 6-311+G**, 6-311++G**, 6-31G(2df), 6-31G(3df), 6-31G(3df,3pd), cc-pVDZ, cc-pVTZ, cc-pVQZ and cc-pV5Z were used in conjunction with various methodologies. A critical comparison of the performance of the density functional methods with MP2, CCSD(T) and G2 methods was made. (C) 2003 Elsevier B.V. All rights reserved.