Orthobaric densities of a square well fluid are obtained by means of a molecular dynamics simulation. Hard wall boundary conditions in one direction and periodic boundary conditions in the other two, added to a mean density close to the critical have two effects: a fast spinodal decomposition that condenses a slab of liquid at the center of the box plus a better sampling of the whole orthobaric curve. Results for the liquid densities compare well with previously obtained data while vapor densities are more sensitive to capillary effects. These finite size effects are analyzed within a lattice gas model, especially those produced by changes in box size and mean density. The method is well suited to predict orthobaric and miscibility curves for binary mixtures once the orthobaric curves of each molecular species have been properly adjusted to experiment.
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