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:

(9)

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:

(10)

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

(11)