The effect of attractive forces on the orderhyphen;disorder phase transition in a gas of long hard rods is examined. The model system is that proposed by Zwanzig and consists of a gas of rectangular parallelepipeds which can orient only in the direction of three perpendicular coordinate axes. We extend Zwanzig's model by including a rectangular parallelpiped of attractive potential which surrounds the hard core in a symmetrical manner. A study is made of the effects of well depth and well width on the parameters characterizing the firsthyphen;order phase transition from an isotropic gas in which the long axes of the rods point in random directions to an anisotropic (ordered) gas. It is observed that even a shallow well (e.g., with a depth of 0.1kT) can yield significant changes in the transition parameters. The results of this study may have implications in the study of nematic ordering in liquid crystalline systems.
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