Isoenthalpic–isobaric ensemble

Statistical mechanics
Thermodynamics Kinetic theory
Particle Statistics Spin-Statistics Theorem Identical Particles Maxwell–Boltzmann Bose–Einstein Fermi–Dirac Parastatistics Anyonic Statistics Braid Statistics
Ensembles NVE Microcanonical NVT Canonical µVT Grand canonical NPH Isoenthalpic–isobaric NPT Isothermal–isobaric µVT Open statistical
Models Debye Einstein Ising
Potentials Internal energy Enthalpy Helmholtz free energy Gibbs free energy Grand potential / Landau free energy
Scientists Maxwell Gibbs Boltzmann Einstein Debye Fermi

The isoenthalpic-isobaric ensemble (constant enthalpy and constant pressure ensemble) is a statistical mechanical ensemble that maintains constant enthalpy $H \,$ and constant pressure $P \,$ applied. It is also called the $NpH$-ensemble, where the number of particles $N \,$ is also kept as a constant. It was developed by physicist H. C. Andersen in 1980. The ensemble adds another degree of freedom, which represents the variable volume $V \,$ of a system to which the coordinates of all particles are relative. The volume $V \,$ becomes a dynamical variable with potential energy and kinetic energy given by $PV \,$. The enthalpy $H=E+PV \,$ is a conserved quantity.