Effects on spectroscopic properties for several low-lying electronic states of CS molecule by core–valence correlation and relativistic corrections

Abstract

The PECs of a 3 Π, a ′ 3 Σ + , d 3 Δ, e 3 Σ − and A 1 Π of CS molecule have been studied using the CASSCF method followed by the MRCI approach with large correlation-consistent basis sets of Dunning and co-workers. Effects on the PECs by the core–valence correlation and relativistic corrections have been taken into account at the level of cc-pV5Z basis set. The way to consider the relativistic corrections is to use the second-order Douglas–Kroll Hamiltonian approximation. Using the CCSD(T), MRCI and MRCI with the Davidson modification, the PECs of electronic states involved are extrapolated to the CBS limit. With these PECs, the spectroscopic parameters and molecular constants of the electronic states involved are determined, which are in excellent agreement with the experimental data. Comparison with the measurements shows that the two-point total-energy extrapolation scheme can obviously improve the quality of spectroscopic parameters and molecular constants. ► Effect on the PECs by the core–valence correlation and relativistic corrections is considered. ► The PECs obtained by the CCSD(T), MRCI and MRCI + Q are extrapolated to the CBS limit. ► The energy extrapolation scheme obviously improves the quality of spectroscopic parameters. ► The spectroscopic parameters and molecular constants obtained achieve much high accuracy. The potential energy curves (PECs) of six low-lying electronic states (X 1 Σ + , a 3 Π, a ′ 3 Σ + , d 3 Δ, e 3 Σ − and A 1 Π) of CS molecule have been investigated using the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach with large correlation-consistent basis sets. Effects on the PECs by the core–valence correlation and relativistic corrections have been taken into account. And the two corrections are performed at the level of cc-pV5Z basis set. The way to consider the relativistic corrections is to use the second-order Douglas–Kroll Hamiltonian approximation. Using the CCSD(T), MRCI and MRCI with the Davidson modification (MRCI + Q), the PECs of electronic states involved are extrapolated to the complete basis set (CBS) limit. With the PECs, the spectroscopic parameters ( T e , R e , ω e , ω e x e , ω e y e , α e , β e , γ e and B e ) of the six low-lying electronic states are determined. These parameters are in excellent agreement with the experimental data. The complete vibrational states are computed for the six low-lying electronic states when the rotational quantum number J equals zero, and the inertial rotation constants of the first 23 vibrational states are reported, which agree favorably with the RKR data. Comparison with the measurements shows that the two-point total-energy extrapolation scheme can obviously improve the quality of spectroscopic parameters and molecular constants.