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tokens: sum except for T = 500 K, which is obtained with the classical approximation. For more details, see the text. From Table 2 , it can be observed that the rotational contributions to the internal partition functions of the four isomers have similar values at the lowest temperatures, e.g. down to T = 10 K. For higher temperatures, above T = 225 K, the difference between the rotational partition function of fulminic acid and the values of the other three isomers (isocyanic acid, cyanic acid, and isofulminic acid), with more similar values, becomes larger. This could be explained because of the structure of fulminic acid is linear and of the other have a near-prolate asymmetric top structure. Nevertheless, the internal partition functions of isocyanic acid, cyanic acid, and isofulminic acid become completely different at higher temperatures despite the fact that these three isomers have a bent structure. This stems from the differences among the vibrational partition functions of the bent isomers. In Table 2 , the weight of the vibrational contribution to the internal partition function can also be assessed for the set of the four CHNO isomers at different temperatures. The differences between the internal partition function and the rotational partition function become noteworthy (more than 1 per cent) at temperatures higher than 225 K for HNCO, 150 K for HOCN, and T = 75 K for HCNO and HONC. Even when the vibrational partition function has been calculated with the harmonic approximation and, in some cases, using fundamentals, the values of this approximation, although underestimated, are in good agreement with the direct sum values (Carvajal et al. 2019 ). As supplementary material, the rotational, vibrational, and rovibrational partition function calculated in the present work from 1 to 500 K (in intervals of 1 K) is also provided for the following isomers: cyanic acid (HOCN); fulminic acid (HCNO); and isofulminic acid (HONC). For isocyanic acid (HNCO), same material can be found elsewhere (Carvajal et al. 2019 ). In Appendix A , we will give some details about the calculation of the partition function for the four CHNO isomers. 4.4 Chemical modelling with <sc>Nautilus</sc> In order to better understand the chemical pathways involved in the production and destruction of HNCO, we used the Nautilus code (Ruaud et al. 2016 ) to carry out a time-dependent chemical model. Nautilus allows to simulate and study grain- and gas-phase chemical processes for instance in hot and cold cores (Semenov et al. 2010 ; Reboussin et al. 2014 ; Ruaud et al. 2015 , 2016 ). The predictions of the model include the time evolution of the chemical abundances for a given set of physical and chemical parameters. For the solid-state chemistry, it considers mantle and surface as chemically active, following the formalism of Hasegawa Herbst ( 1993 ) and the experimental results of Ghesquière et al. ( 2015 ). In this sense, the code can perform a three-phase (gas plus grain