Square-well gas

The second virial coefficient of a hard-sphere gas is positive, illustrating the fact that repulsive forces correspond to a raising of the pressure of the gas over that of an ideal gas at the same molar volume and temperature. The second virial coefficient of the square-well gas has a constant positive part that is identical with that of the hard-sphere gas, and a temperature-dependent negative part due to the attractive part of the potential, illustrating the fact that attractive forces contribute to lowering the pressure of the gas at fixed volume and temperature. 

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