Single partition function for three phases.

The significant structures theory of liquids has been extended to take into account (i) translational degrees of freedom in the degeneracy term and (ii) the perturbation term which becomes important near the critical region. With these improvements, the calculated thermodynamic properties of argon agree very well with experimental results from the melting point through the critical point, along the coexistence curve as well as along the critical isochore and for the solid and the vapor.