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 H-2. Ab initio calculations of the HCN-H-2 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 correlationconsistent triple zeta (aVTZ) basis set. The equilibrium structure is linear HCN-H-2 with the nitrogen pointing towards H-2 at an intermolecular separation of 7.20 a(0). 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 H-2. We also determine the rovibrational energy levels of the HCN-para-H-2 and HCN-ortho-H-2 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-H-2 complex are found to agree by more than 0.5% with the available experimental data, confirming the accuracy of the potential energy surface. (C) 2013 AIP Publishing LLC.