A full-dimensional ab initio intermolecular potential energy surface and ro-vibrational spectra for N₂-HF and N₂-DF

A full-dimensional ab initio intermolecular potential energy surface (IPES) is reported in this paper for van der Waals complex N₂-HF. The calculations were performed by employing the explicitly correlated coupled cluster [CCSD (T)-F12a] method with the augmented correlation-consistent aug-cc-pVTZ basis set plus bond functions. The basis set superposition error was corrected by the full counterpoise procedure. About 55 000 ab initio points were calculated and then fitted by the permutation invariant polynomial neural network approach with a root-mean-square error of 0.433 cm^-1. The potential energy surface features two equivalent linear minima with a well depth of 811.012 cm^-1 separated by a barrier of 635.836 cm^-1. The ro-vibrational energy levels for N₂-HF and N₂-DF were calculated based on the vibrationally averaged 4D IPESs with the radial discrete variable representation/angular finite basis representation method and Lanczos propagation algorithm. The calculated frequencies and the relative line intensities in the HF (DF) stretching band agree well with the available observed spectra. The theoretical band origins are all red shifted relative to the isolated HF (DF) molecule and reproduce the experimental values well. The results of ro-vibrational state calculations demonstrate the high accuracy of our new PES.