A new ab initio potential energy surface for the NH-He complex

We present a new three-dimensional potential energy surface (PES) for the NH(X³Σ^-)-He van der Waals system, which explicitly takes into account the NH vibrational motion. The NH-He PES was obtained using the open-shell single- and double-excitation coupled cluster approach with non-iterative perturbational treatment of triple excitations. The augmented correlation-consistent aug-cc-pVXZ (X = Q, 5, 6) basis sets were employed, and the energies obtained were then extrapolated to the complete basis set limit. Using this new PES, we have studied the spectroscopy of the NH-He complex and we have determined a new rotational constant that agrees well with the available experimental data. Collisional excitation of NH(X³Σ^-) by He was also studied at the close-coupling level. Calculations of the collisional excitation cross sections of the fine-structure levels of NH by He were performed for energies up to 3500 cm^-1, which yield, after thermal average, rate coefficients up to 350 K. The calculated rate coefficients are compared with available experimental measurements at room temperature, and a reasonably good agreement is found between experimental and theoretical data.