The Ω = 4 spin component is the lowest in energy. Individual entries are given for each of the lowest two spin components because 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.
Please not that instead of fixing λ, λ_D, and A_D to zero and obtaining a spin-orbit parameter A nearly 1.7 times that of the ground vibrational state, we fixed these parameters to the ground state values. This yielded an A value marginally different from the ground state value; see
(2) A. Tasa-Chaveli, H. S. P. Müller, Á. Sánchez-Monge, A. Fuente, A. Ginsburg, and T. Möller, 2026, Astrophys. J., accepted.
The calculations may be sufficiently accurate for observational purposes, but some caution is advised for frequencies above 400 GHz amd below about 250 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
(3) 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
(4) R. S. da Silva and M. Y. Ballester, 2022, Mol. Phys. 120, Art. No. e2096142.