Anomaly in the Virial Expansion of IAPWS-95 at Low Temperatures

In the standard reference equation of state for the thermodynamic properties of water, known as IAPWS-95, the fourth virial coefficient D(T) becomes abnormally large in magnitude at temperatures below approximately 300 K. At conditions where a virial expansion using only second and third virial coefficients should be essentially exact (such as vapors at pressures near 100 Pa or 1000 Pa), such a truncated expansion may miss on the order of 2 % of the deviation from ideal-gas behavior in the compressibility factor or the fugacity. The term in IAPWS-95 that causes this issue is identified, and suggestions are made for future equation-of-state development.