Hartree-Fock, post-Hartree-Fock (MP2 and CCSD(T)) and density functional theory (BLYP, B3LYP, BPW91 and B3PW91) methods were employed to obtain the relative stabilities of eight energetically competitive C 2 S 2 H 2 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-ζ 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.