Isobaric ensemble

In such an ensemble we have the reservoir with fixed temperature and pressure. Under these assumptions, the probability that the system has a certain value of energy $E$ $E$ and volume $V$ $V$ is:

P(E,V)=\frac{1}{Z_{P}} g(E,V)e^{-\beta (E+pV)}

Moreover, in such an ensemble we have:

G=-kT\log Z_{P}