2025MNRAS.538.1864B

While the rotational spectroscopy of thiofulminic acid (HCNS), one of the isomers of isothiocyanic acid (HNCS) is established, there is no study of its collisional excitation. In this work, we present the first rate coefficients calculation of the newly observed HCNS($X^{1}\Sigma ^{+}$) colliding with He. To perform this, two-dimensional potential energy surface (2D-PES) is calculated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12] method in conjunction with the aug-cc-pVTZ basis sets. This HCNS-He van der Waals complex's PES has three minima, with the deepest well having a depth of $-72.2$ cm$^{-1}$. Using this PES, inelastic cross-sections were calculated through the close-coupling theory for energies up to 500 cm$^{-1}$ and $j\le 18$. By applying the Maxwell-Boltzmann distribution of kinetic energies, cross-sections were averaged to generate the collisional rate coefficients below 30 K. The latter were compared with those of the valence isoelectronic HCNO and isoelectronic OCS systems with different colliding partner and, very significant differences are found. The new rate coefficients were used to conduct a radiative transfer study and discussion on the excitation behaviour of HCNS was drawn.