Monte Carlo simulations of liquid n-butane

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Abstract
We have carried out Monte Carlo (MC) simulations for a model of liquid n-butane in three thermodynamic states. In each case, both the flexible model in the limit when bond-length and bond-angle forces become infinite and the constrained model were studied. The different trajectories followed by each model lead to different ensemble averages for the intramolecular properties, such as the conformational distribution and intramolecular energy. The changes observed in the external configurational properties, such as the intermolecular energy and pressure are less than the statistical error. In all the cases we have found a weak shift of the trans conformational population with respect to the ideal-gas distribution (i.e. less than 3%). In addition, we have explored the possibilities of a purely repulsive model as a reference system for this liquid. As in the case of simple atomic liquids, the standard hard-core theoretical approaches are good starting points for obtaining the contribution of the external degrees of freedom to the thermodynamic and structural properties. However, repulsive forces shift the conformational equilibrium with respect to the ideal-gas values toward an increase in gauche conformers. The influence of density on internal conformation is more pronounced in the repulsive model than observed for the total potential.