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 N p H -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 P V . The enthalpy H = E + P V is a conserved quantity.
