Ab Initio Path Integral Simulations for the Fluoride Ion-Water Clusters: Competitive Nuclear Quantum Effect Between F--Water and Water-Water Hydrogen Bonds

Small hydrated fluoride ion complexes, F-(H2O) (n = 1-3), have been studied by ab initio hybrid Monte Carlo (HMC) and ab initio path integral hybrid Monte Carlo (PIHMC) simulations. Because of the quantum effect, our simulation shows that the average hydrogen-bonded F-center dot center dot center dot HO distance in the quantum F-(H2O) is shorter than that in the classical one, while the relation inverts at the three water molecular F-(H2O)(3) cluster. In the case of F-(H2O)(3), we have found that the nuclear quantum effect enhances the formation of hydrogen bonds between two water molecules. In F-(H2O)(2) and F-(H2O)(3), the nuclear quantum effect on two different kinds of hydrogen bonds, F--water and water-water hydrogen bonds, competes against each other. In F-(H2O)(3), thus, the nuclear quantum effect on the water-water hydrogen bond leads to the elongation of hydrogen-bonded F-center dot center dot center dot HO distance, which we suggest this as the possible origin of the structural inversion from F-(H2O) to F-(H2O)(3).