A new full-dimensional ab initio intermolecular potential energy surface and vibrational states for (HF)₂ and (DF)₂

We present a new full-dimensional ab initio potential energy surface (PES) of a hydrogen fluoride dimer [(HF)₂] using the supermolecular approach. The calculations were performed at the coupled-cluster single double triple level, with an augmented correlation-consistent polarized valence quadruple-zeta basis set plus bond functions. The basis set superposition error was corrected by a full counterpoise procedure. With the exchange symmetry of the two HF molecules, the permutation invariant polynomial neural network approach was used to fit the hypersurface with a root-mean-square-error of 0.465 cm^-1 for about 110 000 points. The ab initio noise of intermolecular potential in the long range was smoothed by the long-range coefficients method. The equilibrium configuration of the complex was found to be a C_s structure located at two equivalent minima with the well depth of 1573.495 cm^-1. The eigenstates were calculated by employing a symmetry-adapted Lanczos propagation algorithm in the mixed radical discrete variable representation/angular finite basis representation. The tunneling splitting for the ground state of (HF)₂ is 0.665 cm^-1, agreeing well with experimental value of 0.65869 cm^-1. Vibrational fundamentals are also very close to the observed values. The results of vibrational states calculations demonstrate the high accuracy of our new PES.