# Isoenthalpic–isobaric ensemble

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.[1] 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 $PV \,$.[2] The enthalpy $H=E+PV \,$ is a conserved quantity.[3]