Thermodynamics: Residual Properties or Departure functions

Thermodynamic properties like enthalpy and entropy for any substance ideal or real, we can use Maxwell's equations, Thermodynamic Diagrams, property table like saturated steam and superheated steam  or activity coefficient. in addition to these, we can get the approximate values by using Residual Properties which are defined as the difference between the value of thermodynamics property at specified conditions and the thermodynamic property for ideal substance at same specified condition.