Reverse Monte Carlo modelling: the two distinct routes of calculating the experimental structure factor

Abstract: Two different Reverse Monte Carlo strategies, 'RMC++' and 'RMCPOW', have been compared for determining the microscopic structure of some liquid and amorphous solid systems on the basis of neutron diffraction measurements. The first, '$g(r)$ route', exploits the isotropic nature of liquids and calculates the total scattering structure factor, $S(Q)$, via a one-dimensional Fourier transform of the radial distribution function. The second, called 'crystallography' route, is based on the direct calculation of $S(Q)$ in the reciprocal space from the atomic positions in the simulation box. We describe these two methods and apply them to four disordered systems of increasing complexity. The two approaches yield structures in good agreement to the level of two- and three body correlations; consequently, it has been proven that the 'crystallography route' can also deal perfectly with disordered materials. This finding is important for future studies of liquids confined in porous media, where handling Bragg and diffuse scattering simultaneously is unavoidable.