Calculation of thermodynamic properties from an equation of state may appear a trivial problem which only requires knowledge of basic classical thermodynamics. The increasing complexity of thermodynamic models such as the group-contribution lattice-fluid equation of state (GCLF-EOS), however, calls for a systematic approach in order to avoid inefficient or even incorrect computer coded. To generate a fast and thermodynamically consistent computer code for calculation of the thermodynamic properties of mixtures, it is essential to use a modular approach which enables modification of single features of the model, e.g. mixing rule for one of the model parameters without rewriting the entire code. This calls for a formalism where the properties and its derivatives are calculated by combining partial derivatives of the Helmholtz function which ensures a consistent set of relations and leads to an efficient code. This chapter shows how such a procedure ensures that the resulting GCLF-EOS model algorithm will be fully consistent.