Analytical expressions for heat of vaporization are derived by considering soft and hard potentials. Using experimental heat of vaporization along with other physical properties then allows the calculation of the effective hard sphere diameter (EHSD) at different temperatures. The values of EHSD’s of Ar, Xe and CH4 in the range of triple point-ultra boiling point are calculated by using different approaches and the origin of a discrepancy in seff is discussed to some extent. The temperature dependency of EHSD has to be considered, which in turn leads to an expression for the temperature derivative of soft potential ðmsÞ along the line of coexistence between the liquid and vapor phase relating to the Clapeyron–Clausius relation.
