The conventional cubic equation of state (EOS) is analyzed for the purpose of achieving maximum possible precision of representation of thermodynamic functions and parameters of phase equilibrium for pure components, as well as binary and multi-component mixtures. It is derived that the optimal version of EOS for pure components would be 4-parameter equation with size and energy parameters as functions of temperature. The equation fits experimental data on vapor pressure, density and heat of vaporization at the level of precision that is comparable with experimental accuracy, excluding the results at the near-critical range. One-fluid approximation for mixtures is analyzed on general basis of virial theorem. The correction to conventional one-fluid approximation requires introduction of ternary parameters in the expression for energy parameter of mixture, and transforms EOS for mixture from cubic to fifth degree with respect to volume. It is shown both through formulaic derivation and by analysis of experimental data that approaches which ignore this correction can never provide accurate calculations for multi-component mixture on the same level as has been achieved for binary combinations of its constituent components. (C) 2014 Elsevier B.V. All rights reserved.