For astronomically interesting molecules, HCCS and NCS, the equilibrium geometries and potential energy curves of three states (X 2 , A 2 and B 2 Σ + ) as well as vertical excitation energies are studied using complete active space SCF (CASSCF), multi-reference configuration interaction (MRCI) and coupled cluster (CCSD(T)) methods with cc-pVTZ basis sets. The difference and similarity in the three states of HCCS and NCS are illustrated. The results obtained are in good agreement with available experimental data.