A Highly Accurate, Analytic Potential Energy Surface of the Hydrogendifluoride Anion in the Gas Phase

We calculated the full three-dimensional potential energy surface (PES) of an isolated hydrogendifluoride anion (FHF–) in the electronic ground state at a very accurate Coupled Cluster approach and large correlation-consistent valence triple- zeta basis set [CCSD(T)/aug-cc-pVTZ]. The PES was evaluated at more than 30.000 points corresponding to different geometries of the system. Analytical form of the PES was expressed in an internal coordinate set which included the F...F separation (internal coordinate R) and the longitudinal and transversal projection (internal coordinates x and y) of the proton position on the F...F line. For each constant value of x a two-dimensional fit along y and R was performed by using displaced Gaussian functions. The fitted parameters of Gaussians were then spline-interpolated along x to get the final analytical form of the three-dimensional PES. The maximum fitting error was less than 0.01 kcal/mol in the lowest 20 kcal/mol region of the PES, yielding an accurate and conveniently formulated surface which can be readily used for advanced calculations, including fully coupled anharmonic vibrational analysis and quantum dynamics simulation.