A new ab initio potential energy surface for the collisional excitation of HCN by para- and ortho-H2

We present a new four-dimensional potential energy surface for the collisional excitation of HCN by H2. Ab initio calculations of the HCN–H2 van der Waals complex, considering both molecules as rigid rotors, were carried out at the explicitly correlated coupled cluster with single, double, and perturbative triple excitations [CCSD(T)-F12a] level of theory using an augmented correlation-consistent triple zeta (aVTZ) basis set. The equilibrium structure is linear HCN–H2 with the nitrogen pointing towards H2 at an intermolecular separation of 7.20 a0. The corresponding well depth is −195.20 cm−1. A secondary minimum of −183.59 cm−1 was found for a T-shape configuration with the H of HCN pointing to the center of mass of H2. We also determine the rovibrational energy levels of the HCN–para-H2 and HCN–ortho-H2 complexes. The calculated dissociation energies for the para and ortho complexes are 37.79 cm−1 and 60.26 cm−1, respectively. The calculated ro-vibrational transitions in the HCN–H2 complex are found to agree by more than 0.5% with the available experimental data, confirming the accuracy of the potential energy surface