Chemical potential of components in a condensed matter can be expressed using an equilibrium vapor pressure of a coexisting ideal gas even at high pressure. Note that the equilibrium vapor pressure is different from a confining pressure. A confining pressure will be produced by, for example, an inert gas, and the coexisting ideal gas would be a minor component of a confining gas. No gas is of course ideal at high pressures, but we assume that a confining gas is ideal for formulation.
Firstly, we write equation of chemical potential of a condensed matter and its coexisting gas:

where P_A,sup>* is an equilibrium vapor pressure of a component A in a single-component condensed matter at high pressure P. In the case of a multiple-component condensed matter, the equation is then:

where P_A is an equilibrium vapor pressure of a component A in a multiple-component condensed matter at high pressure P.
By using an activity and activity coefficients, we have