The input data set has been reevaluate with respect to the first entry of Feb. 2004. Transitions without hyperfine splitting were taken from
(1) L. Bizzocchi, F. Tonolo, B. M. Giuliano, P. Caselli, M. Melosso, L. Dore, S. Alessandrini, C. Puzzarini, and A. Pietropolli Charmet, 2024, Astrophys. J., 970, Art. No. 26.
Information on the hyperfine splitting was taken from
(2) L. Dore, G. Cazzoli, and P. Caselli, 2001, Astron. Astrophys., 368, 712.
The rest frequencies of (2) were not compatible with those in (1), about 18 kHz too high, and caused a value of the distortion parameter H that was not compatible with values of other isotopologs or a value derived from a potential energy surface in
(3) M. Mladenović, 2017, J. Chem. Phys., 147, Art. No. 114111.
Therefore, only two frequency differences from (2) were fit and H was kept fixed to a value very close to the one from (3) The resulting hyperfine parameters are compatible with those in (1).
The calculation should be sufficiently accurate for observational purposes. Increasing caution is advised above 2 THz.
At low J, it is possible to resolve the ¹⁷O hyperfine splitting. Therefore, a separate hyperfine calculation up to J" = 5 is provided. Note: The partion function given below takes into account this splitting! In addition, they refer to the ground vibrational state only; an estimate of the vibrational partition function of HCO⁺ may be applied.
The dipole moment is assumed to agree with that of the main isotopic species, see e029507.cat.