RMCProfile Utility Programs

Structure refinements using a Reverse Monte Carlo (RMC) method employ large atomic ensembles to fit experimental data. The method relies on a Monte Carlo “random walk” algorithm to identify a configuration that yields the best match to experimental data.  Unlike downhill algorithms used in Rietveld-style refinements, atomic moves that worsen the discrepancy between the calculated and experimental data are accepted but with a probability that decays exponentially as the misfit increases.

RMCProfile is a free software (http://www.isis.rl.ac.uk/RMC/), developed at ISIS, that can fit simultaneously atomic pair-distribution function, total scattering function S(Q), and Bragg profile under various geometric and bond valence sum constraints. Recently, an extension to RMCProfile that enables a simultaneous fit of neutron/X-ray total scattering data, Extended Absorption Fine Structure (EXAFS) (real space), and a geometric shape of diffuse scattering in electron/X-ray single-crystal diffraction was developed at NIST. This refinement algorithm incorporates an explicit treatment of both single- and multiple-scattering contributions to EXAFS and enables accurate modeling of EXAFS over distances encompassing several coordination shells around the absorbing species. More details on the combined refinements and their application to crystalline materials can be found in the following reference: V. Krayzman, I. Levin, J. C. Woicik, Th. Proffen, T. A. Vanderah, and M. G. Tucker, J. Appl. Cryst., 42, (2009). A part related to electron diffuse scattering is still unpublished.

Version 6 of RMCProfile that incorporates combined total scattering/EXAFS/electron diffuse scattering refinements is expected to be posted for download at the ISIS RMC site shortly. In the meantime, the executable (Windows, Unix) can be obtained by contacting Igor Levin at NIST (igor.levin@nist.gov). The manual for a part of RMCProfile dealing with EXAFS and electron diffuse scattering well as the utility programs for calculating electron diffuse scattering (Diffuse) and displacement correlations (AtomCorr) for RMCProfile configurations are available for download.