Partition function (statistical mechanics)

In statistical mechanics, the partition function Z is an important quantity that encodes the statistical properties of a system in thermodynamic equilibrium. It is a function of temperature and other parameters, such as the volume enclosing a gas. Most of the thermodynamic variables of the system, such as the total energy, free energy, entropy, and pressure, can be expressed in terms of the partition function or its derivatives.

There are actually several different types of partition functions, each corresponding to different types of statistical ensemble (or, equivalently, different types of free energy.) The canonical partition function applies to a canonical ensemble, in which the system is allowed to exchange heat with the environment at fixed temperature, volume, and number of particles. The grand canonical partition function applies to a grand canonical ensemble, in which the system can exchange both heat and particles with the environment, at fixed temperature, volume, and chemical potential. Other types of partition functions can be defined for different circumstances.

Suppose we have a thermodynamically large system that is in constant thermal contact with the environment, which has temperature T, with both the volume of the system and the number of constituent particles fixed. This kind of system is called a canonical ensemble. Let us label the exact states (microstates) that the system can occupy by j (j = 1, 2, 3, ...), and denote the total energy of the system when it is in microstate j as E_j. Generally, these microstates can be regarded as discrete quantum states of the system.

The canonical partition function is

${\displaystyle Z=\sum _{j}e^{-\beta E_{j}}}$

where the "inverse temperature" β is conventionally defined as

${\displaystyle \beta \equiv {\frac {1}{k_{B}T}}}$

with k_B denoting Boltzmann's constant. Sometimes degeneracy of states is also used and the partition function becomes

${\displaystyle Z=\sum _{j}g_{j}\cdot e^{-\beta E_{j}}}$,

where ${\displaystyle g_{j}}$ is the degeneracy factor.

In classical statistical mechanics, it is not really correct to express the partition function as a sum of discrete terms, as we have done. In classical mechanics, the position and momentum variables of a particle can vary continuously, so the set of microstates is actually uncountable. In this case, some form of coarse graining procedure must be carried out, which essentially amounts to treating two mechanical states as the same microstate if the differences in their position and momentum variables are "not too large". The partition function then takes the form of an integral. For instance, the partition function of a gas of N classical particles is

${\displaystyle Z={\frac {1}{N!h^{3N}}}\int \,\exp[-\beta H(p_{1}\cdots p_{N},x_{1}\cdots x_{N})]\;\mathrm {d} ^{3}p_{1}\cdots \mathrm {d} ^{3}p_{N}\,\mathrm {d} ^{3}x_{1}\cdots \mathrm {d} ^{3}x_{N}}$

where h is some infinitesimal quantity with units of action (usually taken to be Planck's constant, in order to be consistent with quantum mechanics), and H is the classical Hamiltonian. The reason for the N! factor is discussed below. For simplicity, we will use the discrete form of the partition function in this article, but our results will apply equally well to the continuous form.

In quantum mechanics, the partition function can be more formally written as a trace over the state space (which is independent of the choice of basis):

${\displaystyle Z=\operatorname {tr} (e^{-\beta H})}$

where H is the quantum Hamiltonian operator. The exponential of an operator can be defined, for purely physical considerations, using the exponential power series.

It may not be obvious why the partition function, as we have defined it above, is an important quantity. Firstly, let us consider what goes into it. The partition function is a function of the temperature T and the microstate energies E₁, E₂, E₃, etc. The microstate energies are determined by other thermodynamic variables, such as the number of particles and the volume, as well as microscopic quantities like the mass of the constituent particles. This dependence on microscopic variables is the central point of statistical mechanics. With a model of the microscopic constituents of a system, one can calculate the microstate energies, and thus the partition function, which will then allow us to calculate all the other thermodynamic properties of the system.

The partition function can be related to thermodynamic properties because it has a very important statistical meaning. The probability P_j that the system occupies microstate j is

${\displaystyle P_{j}={\frac {1}{Z}}e^{-\beta E_{j}}.}$

This is the well-known Boltzmann factor. (For a detailed derivation of this result, see canonical ensemble.) The partition function thus plays the role of a normalizing constant (note that it does not depend on j), ensuring that the probabilities add up to one:

${\displaystyle \sum _{j}P_{j}={\frac {1}{Z}}\sum _{j}e^{-\beta E_{j}}={\frac {1}{Z}}Z=1.}$

This is the reason for calling Z the "partition function": it encodes how the probabilities are partitioned among the different microstates, based on their individual energies. The letter Z stands for the German word Zustandssumme, "sum over states".

In order to demonstrate the usefulness of the partition function, let us calculate the thermodynamic value of the total energy. This is simply the expected value, or ensemble average for the energy, which is the sum of the microstate energies weighted by their probabilities:

${\displaystyle \langle E\rangle =\sum _{j}E_{j}P_{j}={\frac {1}{Z}}\sum _{j}E_{j}e^{-\beta E_{j}}=-{\frac {1}{Z}}{\frac {\partial }{\partial \beta }}Z(\beta ,E_{1},E_{2},\cdots )=-{\frac {\partial \ln Z}{\partial \beta }}}$

or, equivalently,

${\displaystyle \langle E\rangle =k_{B}T^{2}{\frac {\partial \ln Z}{\partial T}}.}$

Incidentally, one should note that if the microstate energies depend on a parameter λ in the manner

${\displaystyle E_{j}=E_{j}^{(0)}+\lambda A_{j}\qquad {\mbox{for all}}\;j}$

then the expected value of A is

${\displaystyle \langle A\rangle =\sum _{j}A_{j}P_{j}=-{\frac {1}{\beta }}{\frac {\partial }{\partial \lambda }}\ln Z(\beta ,\lambda ).}$

This provides us with a trick for calculating the expected values of many microscopic quantities. We add the quantity artificially to the microstate energies (or, in the language of quantum mechanics, to the Hamiltonian), calculate the new partition function and expected value, and then set λ to zero in the final expression. This is analogous to the source field method used in the path integral formulation of quantum field theory.

In this section, we will state the relationships between the partition function and the various thermodynamic parameters of the system. These results can be derived using the method of the previous section and the various thermodynamic relations.

As we have already seen, the thermodynamic energy is

${\displaystyle \langle E\rangle =-{\frac {\partial \ln Z}{\partial \beta }}.}$

The variance in the energy (or "energy fluctuation") is

${\displaystyle \langle (\delta E)^{2}\rangle \equiv \langle (E-\langle E\rangle )^{2}\rangle ={\frac {\partial ^{2}\ln Z}{\partial \beta ^{2}}}.}$

The heat capacity is

${\displaystyle C_{v}={\frac {\partial \langle E\rangle }{\partial T}}={\frac {1}{k_{B}T^{2}}}\langle \delta E^{2}\rangle .}$

The entropy is

${\displaystyle S\equiv -k_{B}\sum _{j}P_{j}\ln P_{j}=k_{B}(\ln Z+\beta \langle E\rangle )={\frac {\partial }{\partial T}}(k_{B}T\ln Z)=-{\frac {\partial A}{\partial T}}}$

where A is the Helmholtz free energy defined as A = U - TS, where U=<E> is the total energy and S is the entropy, so that

${\displaystyle A=\langle E\rangle -TS=-k_{B}T\ln Z.}$

Suppose a system is subdivided into N sub-systems with negligible interaction energy. If the partition functions of the sub-systems are ζ₁, ζ₂, ..., ζ_N, then the partition function of the entire system is the product of the individual partition functions:

${\displaystyle Z=\prod _{j=1}^{N}\zeta _{j}.}$

If the sub-systems have the same physical properties, then their partition functions are equal, ζ₁ = ζ₂ = ... = ζ, in which case

${\displaystyle Z=\zeta ^{N}.}$

However, there is a well-known exception to this rule. If the sub-systems are actually identical particles, in the quantum mechanical sense that they are impossible to distinguish even in principle, the total partition function must be divided by a N! (N factorial):

${\displaystyle Z={\frac {\zeta ^{N}}{N!}}.}$

This is to ensure that we do not "over-count" the number of microstates. While this may seem like a strange requirement, it is actually necessary to preserve the existence of a thermodynamic limit for such systems. This is known as the Gibbs paradox.

A specific example of the partition function, expressed in terms of the mathematical formalism of measure theory, is presented in the article on the Potts model.

In a manner similar to the definition of the canonical partition function for the canonical ensemble, we can define a grand canonical partition function for a grand canonical ensemble, a system that can exchange both heat and particles with the environment, which has a constant temperature T, volume V, and chemical potential μ. The grand canonical partition function, although conceptually more involved, simplifies the calculation of the physics of quantum systems. The grand canonical partition function ${\displaystyle {\mathcal {Z}}}$ for an ideal quantum gas is written:

${\displaystyle {\mathcal {Z}}=\sum _{N=0}^{\infty }\,\sum _{\{n_{i}\}}\,\prod _{i}e^{-\beta n_{i}(\epsilon _{i}-\mu )}}$

where N is the total number of particles in the volume V, and index i runs over every microstate of the system, with n_i being the number of particles in state i and ε_i being the energy of state i. {n_i} is the set of all possible occupation numbers for each of the microstates such that Σ_in_i = N.

For example, consider the N = 3 term in the above sum. One possible set of occupation numbers would be {n_i} = 0,1,0,2,0... and the contribution of this set of occupation numbers to the N = 3 term would be

${\displaystyle \prod _{i}e^{-\beta n_{i}(\epsilon _{i}-\mu )}=e^{-\beta (\epsilon _{1}-\mu )}\,e^{-2\beta (\epsilon _{3}-\mu )}.}$

For bosons, the occupation numbers can take any integer values as long as their sum is equal to N. For fermions, the Pauli exclusion principle requires that the occupation numbers only be 0 or 1, again adding up to N.

The above expression for the grand partition function can be shown to be mathematically equivalent to:

${\displaystyle {\mathcal {Z}}=\prod _{i}{\mathcal {Z}}_{i}.}$

(Note: the above product is sometimes taken over all states with equal energy, rather than over each state, in which case the individual partition functions must be raised to a power g_i where g_i is the number of such states. g_i is also referred to as the "degeneracy" of states.)

For a system composed of bosons:

${\displaystyle {\mathcal {Z}}_{i}=\sum _{n_{i}=0}^{\infty }e^{-\beta n_{i}(\epsilon _{i}-\mu )}={\frac {1}{1-e^{-\beta (\epsilon _{i}-\mu )}}}}$

and for a system composed of fermions:

${\displaystyle {\mathcal {Z}}_{i}=\sum _{n_{i}=0}^{1}e^{-\beta n_{i}(\epsilon _{i}-\mu )}=\left(1+e^{-\beta (\epsilon _{i}-\mu )}\right).}$

For the case of a Maxwell-Boltzmann gas, we must use "correct Boltzmann counting" and divide the Boltzmann factor ${\displaystyle e^{-\beta (\epsilon _{i}-\mu )}}$ by n_i!.

${\displaystyle {\mathcal {Z}}_{i}=\sum _{n_{i}=0}^{\infty }{\frac {e^{-\beta n_{i}(\epsilon _{i}-\mu )}}{n_{i}!}}=\exp \left(e^{-\beta (\epsilon _{i}-\mu )}\right).}$

Just as with the canonical partition function, the grand canonical partition function can be used to calculate thermodynamic and statistical variables of the system. As with the canonical ensemble, the thermodynamic quantities are not fixed, but have a statistical distribution about a mean or expected value.

• Defining α=-βμ, the most probable occupation numbers are:

  ${\displaystyle \langle n_{i}\rangle =-\left({\frac {\partial \ln({\mathcal {Z}}_{i})}{\partial \alpha }}\right)_{\beta ,V}={\frac {1}{\beta }}\left({\frac {\partial \ln({\mathcal {Z}}_{i})}{\partial \mu }}\right)_{\beta ,V}.}$

  For Boltzmann particles this yields:

  ${\displaystyle \langle n_{i}\rangle =e^{-(\beta \epsilon _{i}-\mu )}.}$

  For bosons:

  ${\displaystyle \langle n_{i}\rangle ={\frac {1}{e^{\beta (\epsilon _{i}-\mu )}-1}}.}$

  For fermions:

  ${\displaystyle \langle n_{i}\rangle ={\frac {1}{e^{\beta (\epsilon _{i}-\mu )}+1}}.}$

  which are just the results found using the canonical ensemble for Maxwell-Boltzmann statistics, Bose-Einstein statistics and Fermi-Dirac statistics, respectively. (Note: the degeneracy g_i is missing from the above equations because the index i is summing over individual microstates rather than energy eigenvalues.)
• Total number of particles

  ${\displaystyle \langle N\rangle =-\left({\frac {\partial \ln({\mathcal {Z}})}{\partial \alpha }}\right)_{\beta ,V}={\frac {1}{\beta }}\left({\frac {\partial \ln({\mathcal {Z}})}{\partial \mu }}\right)_{\beta ,V}.}$

• Variance in total number of particles

  ${\displaystyle \langle (\delta N)^{2}\rangle =\left({\frac {\partial ^{2}\ln({\mathcal {Z}})}{\partial \alpha ^{2}}}\right)_{\beta ,V}.}$

• Internal energy

  ${\displaystyle \langle E\rangle =-\left({\frac {\partial \ln({\mathcal {Z}})}{\partial \beta }}\right)_{\mu ,V}+\mu \langle N\rangle .}$

• Variance in internal energy

  ${\displaystyle \langle (\delta E)^{2}\rangle =\left({\frac {\partial ^{2}\ln({\mathcal {Z}})}{\partial \beta ^{2}}}\right)_{\mu ,V}.}$

• Pressure

  ${\displaystyle \langle P\rangle ={\frac {1}{\beta }}\left({\frac {\partial \ln({\mathcal {Z}})}{\partial V}}\right)_{\mu ,\beta }.}$

• Mechanical equation of state

  ${\displaystyle \langle PV\rangle ={\frac {\ln({\mathcal {Z}})}{\beta }}.}$

For the case of a non-interacting gas, using the "Semiclassical Approach" we can write (approximately) the inverse of the potential in the form:

${\displaystyle {\frac {1}{2i\pi }}\int _{c-i\infty }^{c+i\infty }\mathrm {d} s{\frac {Z(s)}{{\sqrt {(}}\pi s)}}e^{st}\sim V^{-1}(t)}$

${\displaystyle s={\frac {1}{k_{B}T}}}$ (valid for high T )

supposing that the Hamiltonian of every particle is H=T+V .

Before specific results can be obtained from the grand canonical partition function, the energy levels of the system under consideration need to be specified. For example, the particle in a box model or particle in a harmonic oscillator well provide a particular set of energy levels and are a convenient way to discuss the properties of a quantum fluid. (See the gas in a box and gas in a harmonic trap articles for a description of quantum fluids.)

These results may be used to construct the grand partition function to describe an ideal Bose gas or Fermi gas, and can be used as well to describe a classical ideal gas.

• Huang, Kerson, "Statistical Mechanics", John Wiley & Sons, New York, 1967.
• A. Isihara, "Statistical Physics", Academic Press, New York, 1971.
• Kelly, James J, (Lecture notes)
• L. D. Landau and E. M. Lifshitz, "Statistical Physics, 3rd Edition Part 1", Butterworth-Heinemann, Oxford, 1996.