A computational method of an availability function, which is essential to thermodynamic efficiency analysis of separation processes, is provided. The proposed subroutines of vapor-liquid equilibrium entropy and subsequent availability function are incorporated to the general thermodynamic properties estimation databanks of Fredenslund et al. [1] and Prausnitz et al. [2] The improved databank can be used to calculate availability function of mixtures of up to 20 components made of any combination of 100 species of industrial importance. To illustrate the applicability of the method and the databank, the program is tested to calculate the minimum separation energy in adiabatic and isothermal equilibrium flash distillations for various mixtures. Regardless of close-and wide-boiling mixtures, the availability departures from ideal state are usually significant. Within the limits of thermodynamic frameworks derived here, the calculated ideal availabilities and minimum separation energies show large deviations from the values with nonideality corrections.