Partition Function

The partition function is represented by

where is the thermodynamic coldness and is the energy of the thermodynamic microstate.

The type of partition function depends on the type of thermodynamic ensemble. The thermodynamic ensemble is in turn defined by the thermodynamic parameters which are assumed to be constant throughout the simulation, and is dependent on the properties of the system.

There are generally three types of ensembles that are important in the context of computational chemistry : the Microcanonical Ensemble, the Canonical Ensemble, and the Isotherma-Isobaric Ensemble. These are also called the ensemble, ensemble, and the ensemble, depending on the types of parameters which are kept constant.

A good mental model for the ensembles are as followings

• Microcanonical Ensemble: A thermally sealed canister with a fixed piston at the top.
• Canonical Ensemble: A perfectly thermally conductive cannister with a fixed piston at the top.
• Isotherma-Isobaric Ensemble: A perfectly thermally conductive cannister with a perfectly mobile piston.