The three-term virial equation or a cubic virial equation of state Z = 1 + B ρ ρ + C ρ ρ 2 has the simplicity of the Van der Waals equation of state without its singularity at v = b. Theoretically, the second virial coefficient represents bimolecular attraction forces, and the third virial term represents the repulsive forces among three molecules in close contact.