ϕ = e -β(∆μ(P )) = e -β(µgas(P )-µ water (P )) (6) which allows us to get rid of the reference state of the chemical potential because ϕ only depends on the relative difference of chemical potentials. Consequently, the chemical potential of liquid water is set to zero at the vapor pressure. The fugacity f appearing in Equation is the product of the pressure and the fugacity coefficient: