**Statistical mechanics** is the application of statistics, which includes mathematical tools for dealing with large populations, to the field of Mechanics, which is concerned with the motion of particles or objects when subjected to a force. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties of materials that can be observed in every day life, therefore explaining thermodynamics as a natural result of statistics and mechanics (classical and quantum). In particular, it can be used to calculate the thermodynamic properties of bulk materials from the spectroscopic data of individual molecules.

At the heart of statistical mechanics is the partition function (see Derivation of the partition function):

The partition function provides a measure of the total number of energetic states available to the system at a given temperature. Similarly,

Dividing the second equation by the first equation gives the probability of finding the system in a particular energetic state, :

Helmholtz free energy: | |

Internal energy: | |

Pressure: | |

Entropy: | |

Gibbs free energy: | |

Enthalpy: | |

Constant Volume Heat Capacity: | |

Constant Pressure Heat Capacity: | |

Chemical potential: |

It is often useful to consider the energy of a given molecule to be distributed among a number of modes. For example, translational energy refers to that portion of energy associated with the motion of the center of mass of the molecule. Configurational energy refers to that portion of energy associated with the various attractive and repulsive forces between molecules in a system. The other modes are all considered to be internal to each molecule. They include rotational, vibrational, electronic and nuclear modes. If we assume that each mode is independent (a very questionable assumption!!!!!) the total energy can be expressed as the sum of each of the components:

Expressions for the various molecular partition functions are shown in the following table.

Nuclear | |

Electronic | |

vibrational | |

rotational (linear) | |

rotational (non-linear) | |

Translational | |

Configurational (ideal gas) |

These equations can be combined with those in the first table to determine the contribution of a particular energy mode to a thermodynamic property. For example the "rotational pressure" could be determined in this manner. The total pressure could be found by summing the pressure contributions from all of the individual modes, ie: