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.
