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 ((XII)-I-2, A(2)II and B-2 Sigma(+)) 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. (C) 1997 Elsevier Science B.V.