A relativistic coupled-cluster study on the ytterbium monosulfide molecule (YbS) in synergy with a high resolution microwave spectroscopy study is reported. The quality of the potential energy surfaces and the surfaces of the nuclear electric quadrupole coupling constants (eQq’s) computed using relativistic exact two-component (X2C) coupled-cluster singles and doubles (CCSD), CCSD augmented with a noniterative treatment of triple excitations CCSD(T), and the “Λ”-version of CCSD(T) CCSD(T)Λ is carefully assessed, comparing computed structure parameters and eQq values to precise experimental benchmark values derived from the microwave rotational spectrum of vibrationally excited YbS. The X2C–CCSD calculations are shown to provide qualitatively accurate results, while CCSD(T)Λ calculations provide reliable triples corrections. The inaccuracy of the corresponding CCSD(T) calculations is attributed to a strong variation of orbital rotation due to the coupling between the Yb4f146s0S2s22p6 and Yb4f136s1S2s22p6 configurations along the potential energy surface.
Chen et al. (Mon,) studied this question.