Potential energy surfaces of the ground and low-lying states of HCCS and NCS: CASSCF, MRCI and CCSD(T) studies

For astronomically interesting molecules, HCCS and NCS, the equilibrium geometries and potential energy curves of three states (X2Π, A2Π and B2Σ+) 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.