Determination of the orientational pair correlation function of a molecular liquid from diffraction data

The structure of a molecular liquid is defined by means of the orientational pair correlation function (OPCF), g(r,omega(1),omega(2)), which determines the likelihood of a molecule being found at position r, with orientation wt, given a molecule with orientation or at the origin. Diffraction experiments and most computer simulations of liquids however present only the site-site radial distribution functions, which by definition contain less information than the OPCF. By expanding the OPCF as a series of generalised spherical harmonic functions (SHARM), and by showing that this series, even when truncated, contains important orientational information, it is proposed that the coefficients to this series should provide the essential database for the structure of a molecular liquid as derived from a given set of diffraction data. The most reliable method of estimating these coefficients is to run an auxilliary computer simulation which incorporates all known physical constraints on molecular positions as well as reproducing the measured radial distribution functions or diffraction data as accurately as possible. The Empirical Potential Structure Refinement (EPSR) method of performing the simulation is a robust method of obtaining these structural coefficients and the results appear to be relatively insensitive to the details of the assumed reference intermolecular potential. The possibility of using the EPSR technique as a route to establishing an accurate intermolecular potential is also discussed. Some examples of applying the method to liquid water are presented. (C) 1998 Elsevier Science B.V. All rights reserved.