  An extension of the well established Reverse Monte Carlo (RMC) method for
modeling systems under close confinement has been developed. The method
overcomes limitations induced by close confinement in systems such as fluids
adsorbed in microporous materials. As a test of the method, we investigate a
model system of $^{36}$Ar adsorbed into two zeolites with significantly
different pore sizes: Silicalite-I (a pure silica form of ZSM-5 zeolite,
characterized by relatively narrow channels forming a 3D network) at partial
and full loadings and siliceous Faujasite (which exhibits relatively wide
channels and large cavities). The model systems are simulated using Grand
Canonical Monte Carlo and, in each case, its structure factor is used as input
for the proposed method, which shows a rapid convergence and yields an
adsorbate microscopic structure in good agreement with that of the model
system, even to the level of three body correlations, when these are induced by
the confining media. The application to experimental systems is straightforward
incorporating factors such as the experimental resolution and appropriate
q-sampling, along the lines of previous experiences of RMC modeling of powder
diffraction data including Bragg and diffuse scattering.
