The concept of molecular-thermodynamic reference equations of state (M
TREOS) for developing a molecular-based equation of state is introduce
d. A MTREOS may refer to an artificial ''system'' and must therefore b
e distinguished from the system defined by a reference intermolecular
potential u0. Possible MTREOS for various classes of fluids are discus
sed. The concept is then exemplified by investigating the Lennard-Jone
s fluid. Two different examples of a MTREOS, given by the hard sphere
and molecular-based correction terms, are defined and assessed, and a
closed analytical expression for the reference system pressure is deri
ved.