Isoenthalpic–isobaric ensemble

The isoenthalpic-isobaric ensemble (constant enthalpy and constant pressure ensemble) is a statistical mechanical ensemble that maintains constant enthalpy ${\displaystyle H\,}$ and constant pressure ${\displaystyle P\,}$ applied. It is also called the ${\displaystyle NPH}$-ensemble, where the number of particles ${\displaystyle N\,}$ 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 ${\displaystyle V\,}$ of a system to which the coordinates of all particles are relative. The volume ${\displaystyle V\,}$ becomes a dynamical variable with potential energy and kinetic energy given by ${\displaystyle PV\,}$. The enthalpy ${\displaystyle H=E+PV\,}$ is a conserved quantity. Using isoenthalpic-isobaric ensemble of Lennard-Jones fluid, it was shown that the Joule–Thomson coefficient and inversion curve can be computed directly from a single molecular dynamics simulation. A complete vapor-compression refrigeration cycle and a vapor–liquid coexistence curve, as well as a reasonable estimate of the supercritical point can be also simulated from this approach. NPH simulation can be carried out using GROMACS and LAMMPS.