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# Isothermal-isobaric ensemble

The **isothermal-isobaric ensemble** has the following variables:

- is the number of particles
- is the pressure
- is the temperature

The classical partition function, for a one-component atomic system in 3-dimensional space, is given by

where

- is the Volume:

- , where is the Boltzmann constant

- represent the reduced position coordinates of the particles; i.e.

- is the potential energy, which is a function of the coordinates (or of the volume and the reduced coordinates)