# Statistical mechanics

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 everyday life, therefore explaining thermodynamics as a natural result of statistics and mechanics (classical and quantum) at the microscopic level. In particular, it can be used to calculate the thermodynamic properties of bulk materials from the spectroscopic data of individual molecules.

This ability to make macroscopic predictions based on microscopic properties is the main asset of statistical mechanics over thermodynamics. Both theories are governed by the second law of thermodynamics through the medium of entropy. However, Entropy in thermodynamics can only be known empirically, whereas in Statistical mechanics, it is a function of the distribution of the system on its micro-states.

## Microcanonical ensemble

Since the second law of thermodynamics applies to isolated systems, the first case investigated will correspond to this case. The Microcanonical ensemble describes an isolated system.

The entropy of such a system can only increase, so that the maximum of its entropy corresponds to an equilibrium state for the system.

Because an isolated systems keeps a constant energy, the total energy of the system does not fluctuate. Thus, the system can access only those of its micro-states that correspond to a given value E of the energy. The internal energy of the system is then strictly equal to its energy.

Let us call $\displaystyle ''\Omega(E)$ the number of micro-states corresponding to this value of the system's energy. The macroscopic state of maximal entropy for the system is the one in which all micro-states are equally likely to occur during the system's fluctuations.

$\displaystyle S=k_B\ln \left(\Omega (E) \right) \,$
where
$\displaystyle S$ is the system entropy,
$\displaystyle k_B$ is Boltzmann's constant

## Canonical ensemble

Invoking the concept of the canonical ensemble, it is possible to derive the probability $\displaystyle P_i$ that a macroscopic system in thermal equilibrium with its environment will be in a given microstate with energy $\displaystyle E_i$ :

$\displaystyle P_i = {\exp\left(-\beta E_i\right)\over{\sum_j^{j_{max}}\exp\left(-\beta E_j\right)}}$
where $\displaystyle \beta={1\over{kT}}$ ,

The temperature T arises from the fact that the system is in thermal equilibrium with its environment . The probabilities of the various microstates must add to one, and the normalization factor in the denominator is the canonical partition function:

$\displaystyle Z = \sum_j^{j_{max}} \exp\left(-\beta E_j\right)$

where $\displaystyle E_i$ is the energy of the ith microstate of the system. The partition function is a measure of the number of states accessible to the system at a given temperature. See derivation of the partition function for a proof of Boltzmann's factor and the form of the partition function from first principles.

To sum up, the probability of finding a system at temperature T in a particular state with energy Ei is

$\displaystyle P_i = \frac{\exp(-\beta E_i)}{Z}$

### Thermodynamic Connection

The partition function can be used to find the expected (average) value of any microscopic property of the system, which can then be related to macroscopic variables. For instance, the expected value of the microscopic energy E is interpreted as the microscopic definition of the thermodynamic variable internal energy (U)., and can be obtained by taking the derivative of the partition function with respect to the temperature. Indeed,

$\displaystyle \langle E\rangle={\sum_i E_i e^{-\beta E_i}\over Z}=-{dZ\over d\beta}/Z$

implies, together with the interpretation of <E> as U, the following microscopic definition of internal energy:

$\displaystyle U\colon = -{d\ln Z\over d \beta}.$

The entropy can be calculated by (see Shannon entropy)

$\displaystyle {S\over k} = - \sum_i p_i \ln p_i = \sum_i {e^{-\beta E_i}\over Z}(\beta E_i+\ln Z) = \ln Z + \beta U$

which implies that

$\displaystyle -\frac{\ln(Z)}{\beta} = U - TS = F$

is the Free energy of the system or in other words,

$\displaystyle Z=e^{-\beta F}\,$

Having microscopic expressions for the basic thermodynamic potentials U (internal energy), S (entropy) and F (free energy) is sufficient to derive expressions for other thermodynamic quantities. The basic strategy is as follows. There may be an intensive or extensive quantity that enters explicitly in the expression for the microscopic energy Ei, for instance magnetic field (intensive) or volume (extensive). Then, the conjugate thermodynamic variables are derivatives of the internal energy. For instance, the macroscopic magnetization (extensive) is the derivative of U with respect to the (intensive) magnetic field, and the pressure (intensive) is the derivative of U with respect to volume (extensive).

The treatment in this section assumes no exchange of matter (i.e. fixed mass and fixed particle numbers). However, the volume of the system is variable which means the density is also variable.

This probability can be used to find the average value, which corresponds to the macroscopic value, of any property, $\displaystyle J$ , that depends on the energetic state of the system by using the formula:

$\displaystyle \langle J \rangle = \sum_i p_i J_i = \sum_i J_i \frac{\exp(-\beta E_i)}{Z}$

where $\displaystyle $ is the average value of property $\displaystyle J$ . This equation can be applied to the internal energy, $\displaystyle U$ :

$\displaystyle U = \sum_i E_i \frac{\exp(\beta E_i)}{Z}$

Subsequently, these equations can be combined with known thermodynamic relationships between $\displaystyle U$ and V to arrive at an expression for pressure in terms of only temperature, volume and the partition function. Similar relationships in terms of the partition function can be derived for other thermodynamic properties as shown in the following table.

 Helmholtz free energy: $\displaystyle F = - {\ln Z\over \beta}$ Internal energy: $\displaystyle U = -\left( \frac{\partial\ln Z}{\partial\beta} \right)_{N,V}$ Pressure: $\displaystyle P = -\left({\partial F\over \partial V}\right)_{N,T}= {1\over \beta} \left( \frac{\partial \ln Z}{\partial V} \right)_{N,T}$ Entropy: $\displaystyle S = k (\ln Z + \beta U)\,$ Gibbs free energy: $\displaystyle G = F+PV=-{\ln Z\over \beta} + {V\over \beta} \left( \frac{\partial \ln Z}{\partial V}\right)_{N,T}$ Enthalpy: $\displaystyle H = U + PV\,$ Constant Volume Heat capacity: $\displaystyle C_V = \left( \frac{\partial U}{\partial T} \right)_{N,V}$ Constant Pressure Heat capacity: $\displaystyle C_P = \left( \frac{\partial U}{\partial T} \right)_{N,P}$ Chemical potential: $\displaystyle \mu_i = -{1\over \beta} \left( \frac{\partial \ln Z}{\partial N_i} \right)_{T,V,N}$

The last entry needs clarification. We are NOT working with a grand canonical ensemble here.

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 questionable assumption) the total energy can be expressed as the sum of each of the components:

$\displaystyle E = E_t + E_c + E_n + E_e + E_r + E_v\,$

Where the subscripts t, c, n, e, r, and v correspond to translational, configurational, nuclear, electronic, rotational and vibrational modes, respectively. The relationship in this equation can be substituted into the very first equation to give:

$\displaystyle Z = \sum_i \exp\left(-\beta(E_{ti} + E_{ci} + E_{ni} + E_{ei} + E_{ri} + E_{vi})\right)$
$\displaystyle = \sum_i \exp\left(-\beta E_{ti}\right) \exp\left(-\beta E_{ci}\right) \exp\left(-\beta E_{ni}\right) \exp\left(-\beta E_{ei}\right) \exp\left(-\beta E_{ri}\right) \exp\left(-\beta E_{vi}\right)$

If we can assume all these modes are completely uncoupled and uncorrelated, so all these factors are in a probability sense completely independent, then

$\displaystyle Z = Z_t Z_c Z_n Z_e Z_r Z_v\,$

Thus a partition function can be defined for each mode. Simple expressions have been derived relating each of the various modes to various measurable molecular properties, such as the characteristic rotational or vibrational frequencies.

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

 Nuclear $\displaystyle Z_n = 1 \qquad (T < 10^8 K)$ Electronic $\displaystyle Z_e = W_0 \exp(kT D_e + W_1 \exp(-\theta_{e1}/T) + \cdots)$ vibrational $\displaystyle Z_v = \prod_j \frac{\exp(-\theta_{vj} / 2T)}{1 - \exp(-\theta_{vj} / T)}$ rotational (linear) $\displaystyle Z_r = \frac{T}{\sigma} \theta_r$ rotational (non-linear) $\displaystyle Z_r = \frac{1}{\sigma}\sqrt{\frac{{\pi}T^3}{\theta_A \theta_B \theta_C}}$ Translational $\displaystyle Z_t = \frac{(2 \pi mkT)^{3/2}}{h^3}$ Configurational (ideal gas) $\displaystyle Z_c = V\,$

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:

$\displaystyle P = P_t + P_c + P_n + P_e + P_r + P_v\,$

## Grand canonical ensemble

If the system under study is an open system, (matter can be exchanged), and particle number is conserved, we would have to introduce chemical potentials, μj, j=1,...,n and replace the canonical partition function with the grand canonical partition function:

$\displaystyle \Xi(V,T,\mu) = \sum_i \exp\left(\beta \left[\sum_{j=1}^n \mu_j N_{ij}-E_i\right ]\right)$

where Nij is the number of jth species particles in the ith configuration. Sometimes, we also have other variables to add to the partition function, one corresponding to each conserved quantity. Most of them, however, can be safely interpreted as chemical potentials. In most condensed matter systems, things are nonrelativistic and mass is conserved. However, most condensed matter systems of interest also conserve particle number approximately (metastably) and the mass (nonrelativistically) is none other than the sum of the number of each type of particle times its mass. Mass is inversely related to density, which is the conjugate variable to pressure. For the rest of this article, we will ignore this complication and pretend chemical potentials don't matter. See grand canonical ensemble.

Let's rework everything using a grand canonical ensemble this time. The volume is left fixed and does not figure in at all in this treatment. As before, j is the index for those particles of species j and i is the index for microstate i:

$\displaystyle U = \sum_i E_i \frac{\exp(-\beta (E_i-\sum_j \mu_j N_{ij}))}{\Xi}$
$\displaystyle N_j = \sum_i N_{ij} \frac{\exp(-\beta (E_i-\sum_j \mu_j N_{ij}))}{\Xi}$
 Gibbs free energy: $\displaystyle G = - {\ln \Xi\over \beta}$ Internal energy: $\displaystyle U = -\left( \frac{\partial\ln \Xi}{\partial\beta} \right)_{\mu}+\sum_i{\mu_i\over\beta}\left({\partial \ln \Xi\over \partial \mu_i}\right )_{\beta}$ Particle number: $\displaystyle N_i={1\over\beta}\left({\partial \ln \Xi\over \partial \mu_i}\right)_\beta$ Entropy: $\displaystyle S = k (\ln \Xi + \beta U- \beta \sum_i \mu_i N_i)\,$ Helmholtz free energy: $\displaystyle F = G+\sum_i \mu_i N_i=-{\ln \Xi\over \beta} +\sum_i{\mu_i\over \beta} \left( \frac{\partial \ln \Xi}{\partial \mu_i}\right)_{\beta}$

## Equivalence between descriptions at the thermodynamic limit

All the above descriptions differ in the way they allow the given system to fluctuate between its configurations.

In the micro-canonical ensemble, the system exchanges no energy with the outside world, and is therefore not subject to energy fluctuations, while in the canonical ensemble, the system is free to exchange energy with the outside in the form of heat.

In the thermodynamic limit, which is the limit of large systems, fluctuations become negligible, so that all these descriptions converge to the same description. In other words, the macroscopic behavior of a system does not depend on the particular ensemble used for its description.

Given these considerations, the best ensemble to choose for the calculation of the properties of a macroscopic system is that ensemble which allows the result be most easily derived.

A Table of Statistical Mechanics Articles
Maxwell Boltzmann Bose-Einstein Fermi-Dirac
Particle Boson Fermion
Statistics
Statistics Bose-Einstein statistics Fermi-Dirac statistics
Thomas-Fermi
approximation
gas in a box
gas in a harmonic trap
Gas Ideal gas
Chemical
Equilibrium
Classical Chemical equilibrium