The Ω = 4 spin component is the lowest in energy. Individual entries are given for each of the lowest two spin components because Ω = 2 is perturbed, and the perturber is not known. Moreover, spfit has difficulty treating the λ-doubling in FeS completely within uncertainties. The lowest two spin-orbit components were reproduced well, and the energies and frequencies of the remaining two components are probably reasonable.
The lines were taken from
(1) S. Takano, S. Yamamoto, and S. Saito, 2004, J. Mol. Spectrosc. 224, 137.
The calculations may be sufficiently accurate for observational purposes, but some caution is advised for frequencies above 400 GHz amd below about 150 GHz.
The partition function takes into account all spin-orbit components approximately. However, it refers to the ground vibrational state only. Vibrational corrections may be estimated from its ground state harmonic vibration of 518 ± 5 cm^–1 determined by
(2) L. Wang, D.-L. Huang, J.-F. Zhen, Q. Zhang, and Y. Chen, 2011, Chin. J. Chem. Phys. 24, 1.
The anharmonicity or the low-lying A state were not taken into account because of the relatively large uncertainty of the harmonic vibration.
The dipole moment was taken from a quantum chemical calculation by
(3) R. S. da Silva and M. Y. Ballester, 2022, Mol. Phys. 120, Art. No. e2096142.