Isothermal–isobaric ensemble

Summary

The isothermal–isobaric ensemble, or $NPT$ ensemble, describes a system with a fixed number $N$ of particles at fixed temperature $T$ and fixed pressure $P$. Its volume can change, as for a gas under a freely moving piston. A state with energy $E$ and volume $V$ gets weight proportional to $e^{-(E + PV) / (k_B T)}$. Its partition function gives the Gibbs free energy, which governs chemistry and phase changes at constant pressure.