where V is the volume of the system and ϕ the fugacity coefficient at pressure P . The chemical potential is related to the pressure of the reservoir by µ water (P ) relationship, and it is more convenient to use pressure rather than chemical potential as input in a GCMC simulation, because chemical potential is defined from an arbitrary reference state. The fugacity coefficient ϕ appearing in Equation is the Boltzmann factor of the difference in chemical potential between the gas phase and the liquid phase denoted by ∆μ(P ) in Figure at a given pressure P :