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In this work we present a new approach to produce spectroscopic constants and model first-principles synthetic spectra for all molecules of astrophysical interest. We have generalized our previous diatomic molecule simulation framework, employing Transition-Optimised Shifted Hermite (TOSH) theory, thereby enabling the modelling of polyatomic rotational constants for molecules with three or more atoms. These capabilities, are now provided by our new code Epimetheus. As a first validation of our approach, we confront our predictions and assess their accuracy against the well-studied triatomic molecule, ozone 666 ($^{16}$O$_3$), in addition to eight of its potential isotopomers: ozone 668 ($^{16}$O$^{16}$O$^{18}$O), 686 ($^{16}$O$^{18}$O$^{16}$O), 667 ($^{16}$O$^{16}$O$^{17}$O), 676 ($^{16}$O$^{17}$O$^{16}$O), 688 ($^{16}$O$^{18}$O$^{18}$O), 868 ($^{18}$O$^{16}$O$^{18}$O), 888 ($^{18}$O$_3$), and 777 ($^{17}$O$_3$). We then assess the accuracy of these rotational constants using the Epimetheus data in our code Pandora, and generate synthetic molecular spectra. The ozone spectra presented here are purely infrared and not Raman. Epimetheus builds upon the work from our previous code Prometheus, which used the TOSH theory to account for anharmonicity for the fundamental $\nu=0 \rightarrow \nu=1$ band, going further to now account for triatomic molecules. This is combined with thermal profile modeling for the rotational transitions. We have found that this extended method performs promisingly, typically approximating the spectroscopic constants and spectra well. Some issues do arise depending on the symmetry group of the ozone isotopomer. In general, we show that Epimetheus can provide the data to produce appreciable molecular spectra, to help drive future high-resolution studies., Comment: 26 pages, 6 figure, 10 tables -- Submitted to ApJ; comments from the community welcomed!

This work presents the first steps to modelling synthetic rovibrational spectra for all molecules of astrophysical interest using a new approach implemented in the Prometheus code. The goal is to create a new comprehensive source of first-principles molecular spectra, thus bridging the gap for missing data to help drive future high-resolution studies. Our primary application domain is for molecules identified as signatures of life in planetary atmospheres (biosignatures), but our approach is general and can be applied to other systems. In this work we evaluate the accuracy of our method by studying four diatomic molecules H$_2$, O$_2$, N$_2$ and CO, all of which have well-known spectra. Prometheus uses the Transition-Optimised Shifted Hermite (TOSH) theory to account for anharmonicity for the fundamental $\nu=0 \rightarrow \nu=1$ band, along with thermal profile modeling for the rotational transitions. To this end, we expand TOSH theory to enable the modeling of rotational constants. We show that our simple model achieves results that are a better approximation of the real spectra than those produced through a harmonic approach. We compare our results with high-resolution HITRAN and ExoMol spectral data. We find that modelling accuracy tends to diminish for rovibrational transition away from the band origin, thus highlighting the need for the theory to be further adapted., Comment: 29 pages, 9 figures, 1 table, accepted for publication in The Astrophysical Journal