Robust Performance of the Noniterative Triples Corrections in CCSD(T) _Λ : YbS as a Challenging Case with Strong Variation of Orbital Rotation

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 Yb[4f146s0]S[2s22p6] and Yb[4f136s1]S[2s22p6] configurations along the potential energy surface.