Abstract :
It is shown that data on the site-site pair correlation functions for a fluid of molecules can be used to derive a set of empirical site-site potential energy functions. These potential functions reproduce the fluid structure accurately but at the present time do not reproduce thermodynamic information on the fluid, such as the internal energy or pressure. The method works in an iterative manner, starting from a reference fluid in which only Lennard-Jones interactions are included, and generates, by Monte Carlo simulation, successive corrections to those potentials which eventually lead to the correct site-site pair correlation functions. Using the approach the structure of water as determined from neuron scattering experiments is compared to the structure of water obtained from the simple point charge extended (SPCE) model of water interactions. The empirical potentials derived from both experiment and SPCE water show qualitative similarities with the true SPCE potential, although there are quantitative differences. The simulation is driven by a set of potential energy functions, with equilibration of the energy of the distribution, and not, as in the reverse Monte Carlo method, by equilibrating the value of χ2, which measures how closely the simulated site-site pair correlation functions fit a set of diffraction data. As a result the simulation proceeds on a true random walk and samples a wide range of possible molecular configurations.
