Improving the point-charge description of hydrogen bonds by adaptive force matching

The MP2f water force field, recently developed using the adaptive force matching method, is improved significantly after the introduction of a short-range repulsion term. This term is designed to address the deficiency of point-charge models at short charge-charge separations. It may also provide a more realistic description of hydrogen bonds by capturing exchange-repulsion interactions. When compared with MP2f, the new MP2f_hb force field predicts properties of liquid water in much better agreement with experiments. The calculated site-site radial distribution functions are in good agreement with X-ray diffraction data corroborating the argument that point-charge models with only Lennard-Jones repulsion underestimate the nearest neighbor O-O distance. The incorporation of the new short-range repulsion term is argued to be important for any force field model that uses point charges to model electrostatics.