  We present a systematic approach to construct complete equations of state
(EOSs), or to ensure thermodynamic consistency of complete and incomplete forms
of EOSs using a minimal and sufficient set of relations. We apply the proposed
approach to some common classical equations of state for pure materials. In
fact, classical equations of state come generally in an incomplete form that
hides essencial properties necessary for thermodynamic consistency. If not
aware of such constraints one may generalize the EOS, or fit its thermodynamic
parameters from emprirical data in an inconsistent way.
