Collisional dynamics of NC_4_NH^+^ with H_2_

Such low-temperature collisions help determine the state-to-state rate coefficients for the rotational transitions of NC_4_NH^+^, a recently detected species in TMC-1. To study these collisions, a four-dimensional (4D) ab initio potential energy surface (PES) for the NC_4_NH^+^-H_2_ system was developed using the CCSD(T)-F12b method with an augmented triple-zeta basis set under the rigid-rotor approximation.

The PES was then augmented through a neural network (NN) fitting approach to get the dense data. It was further expanded in bispherical harmonics, for the radial coefficients to be expressed in analytical form. Based on these radial coefficients, state-to-state cross sections and rate coefficients were calculated using the exact close-coupling method for the rotational states considering to both para- and ortho-H_2_.