A molecular-based theory for the thermodynamic properties of water

Following the rules set by the molecular theories of fluids, a perturbed fo rm of the Helmholtz free energy for water has been developed. The reference term corresponds to short range water, and is approximated by the properti es of a primitive model; the perturbation term is given by contributions of the dispersion forces and the dipole-dipole interaction. The method is fir st verified by applying it to TIP4P water and then used for real water with out reference to any specific potential. The parameters of the model are de termined in order to obtain the best representation of the vapour pressure and coexistence liquid densities from the triple point to 643.15 K; no atte mpt is made to rt the critical region. Despite a number of approximations e mployed, the accuracy of the equation of state is comparable with that of t he modified Redlich-Kwong-Soave equation and SAFT Yukawa-dipole-dipole equa tion, and considerably better than the accuracy of SAFT-HS and SAFT-VR equa tions. Because of its true molecular footing, the equation remains reliable also for various thermodynamic properties outside the coexistence region. It reproduces the anomaly in the isothermal compressibility, locating its m inimum at T = 38 degreesC (versus the experimental value T = 46 degreesC) f or P = 1 bar. It also predicts a density maximum, but outside the experimen tal temperature range (at temperatures below the triple-point temperature).