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.