On the Uniqueness of the Reverse Monte Carlo Simulation for Molecular Liquids

Abstract

A careful analysis of the three dimensional structures of liquid Chlorine produced by the Reverse Monte Carlo (RMC) and Molecular Dynamics (MD) techniques is presented. The analysis allows us to measure the degree of uniqueness between the potential and the atom-atom distribution functions, g _aa(r), in the case of pairwise potentials formed by isotropic and anisotropic site-site interactions. The g _aa(r) obtained from MD simulations are used as ‘experimental’ input data in the RMC procedure and the constraint of rigid molecules is imposed. The particle configurations produced by RMC are then studied by using a recently proposed general method for analysing the local order in liquids. The same analysis applied to the particle configurations produced by the conventional MD simulation yields a set of partial distribution functions which relates the main features of the g _aa(r) to microscopic pair geometries. The comparison between the partial centre-centre g _cc(r) shows that the three dimensional structures, produced by MD and RMC simulations, agree very well when only isotropic site-site interactions act. In this case RMC produces the same radial distribution function g(r, ω₁, ω₂) as that obtained from the original MD configurations; it is therefore a valid tool for deriving a complete information on the physical properties of a fluid. For anisotropic site-site interactions the partial g _cc(r) of MD and RMC differ significantly and show that the three dimensional structures, produced by MD and RMC simulations, differ too. The discrepancies are particularly evident for the T shaped configurations and affect the values of the potential energy. Therefore, even if the potential is purely pairwise additive, the use of the atomic radial distribution function as input data and the imposition of atomic constraints which model the molecules as hard dumbbells are not sufficient to bring the RMC procedure towards the ‘true’ microscopic structure of the liquid; the presence of non central forces between sites disrupts the bijective correspondence between the potential and the g _aa(r).