The reverse Monte Carlo technique applied to fluids of diatomic molecules

A careful analysis of the three-dimensional structures of liquid iodine obtained with the reverse Monte Carlo (RMC) and molecular dynamics (MD) techniques is presented. The analysis allows one to measure the degree of nonuniqueness between the potential and the radial distribution functions g(r), in the case of pairwise but not purely site-site interactions. The g(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(r) to microscopic pair geometries. A comparison of the partial g(r) shows that the three-dimensional structures produced by the MD and RMC simulations differ significantly. In other words, 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 is not sufficient to bring the RMC procedure towards the 'true' microscopic structure of the liquid. The discrepancies are particularly evident for the elongated configurations, such as the T, L and end-to-end. The use of the centres of mass distribution functions as additional input data does not yield significant improvements in the microscopic local order predicted by the RMC simulation.