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Partition function (statistical mechanics)

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In statistical mechanics, the '''partition function''' ''Z'' is an important quantity that encodes the statistics statistical properties of a system in thermodynamic equilibrium. It is a function (mathematics) function of temperature and other parameters, such as the volume enclosing a gas. Most of the thermodynamics thermodynamic variables of the system, such as the energy 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.

Canonical partition function


Definition
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 : $Z\; =\; \backslash sum\_\{j\}\; e^\{-\; \backslash beta\; E\_j\}$ where the "inverse temperature" ''β'' is conventionally defined as : $\backslash beta\; \backslash equiv\; \backslash frac\{1\}\{k\_BT\}$ with ''k_B'' denoting Boltzmann's constant. 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 set 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 : $Z=\backslash frac\{1\}\{N!h^\{3N\}\}\; \backslash int\; \backslash ,\; \backslash exp[-\backslash beta\; H(p\_1\; \backslash cdots\; p\_N,\; x\_1\; \backslash cdots\; x\_N)]\; \backslash ;\; d^3p\_1\; \backslash cdots\; d^3p\_N\; \backslash ,\; d^3x\_1\; \backslash cdots\; d^3x\_N$ where ''h'' is some infinitesimal quantity with units of action (physics) action (usually taken to be Planck's constant, in order to be consistent with quantum mechanics), and ''H'' is the classical Hamiltonian mechanics Hamiltonian. The reason for the ''N''! factor is discussed #Partition functions of subsystems 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 (linear algebra) trace over the mathematical formulation of quantum mechanics state space (which is independent of the choice of basis (linear algebra) basis): : $Z=\backslash operatorname\{tr\}\; (\; e^\{-\backslash beta\; H\}\; )$ where ''H'' is the Hamiltonian (quantum mechanics) quantum Hamiltonian operator. Note that the exponential of an operator is defined using the Characterizations of the exponential function usual power series.

Meaning and significance
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 ''E''₁, ''E''₂, ''E''₃, 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 : $P\_j\; =\; \backslash frac\{1\}\{Z\}\; e^\{-\; \backslash beta\; E\_j\}.$ This is the well-known Boltzmann factor. (For a detailed derivation of this result, see Derivation of the partition function.) 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: : $\backslash sum\_j\; P\_j\; =\; \backslash frac\{1\}\{Z\}\; \backslash sum\_j\; e^\{-\; \backslash beta\; E\_j\}\; =\; \backslash frac\{1\}\{Z\}\; Z\; =\; 1.$ 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 language German word ''Zustandssumme'', "sum over states".

Calculating the thermodynamic total energy
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: : $\backslash langle\; E\; \backslash rangle\; =\; \backslash sum\_j\; E\_j\; P\_j\; =\; \backslash frac\{1\}\{Z\}\; \backslash sum\_j\; E\_j\; e^\{-\; \backslash beta\; E\_j\}\; =\; -\; \backslash frac\{1\}\{Z\}\; \backslash frac\{\backslash partial\}\{\backslash partial\; \backslash beta\}\; Z(\backslash beta,\; E\_1,\; E\_2,\; \backslash cdots)\; =\; -\; \backslash frac\{\backslash partial\; \backslash ln\; Z\}\{\backslash partial\; \backslash beta\}$ or, equivalently, : $\backslash langle\; E\backslash rangle\; =\; k\_B\; T^2\; \backslash frac\{\backslash partial\; \backslash ln\; Z\}\{\backslash partial\; T\}.$ Incidentally, one should note that if the microstate energies depend on a parameter λ in the manner : $E\_j\; =\; E\_j^\{(0)\}\; +\; \backslash lambda\; A\_j\; \backslash qquad\; \backslash mbox\{for\; all\}\backslash ;\; j$ then the expected value of ''A'' is : $\backslash langle\; A\backslash rangle\; =\; \backslash sum\_j\; A\_j\; P\_j\; =\; -\backslash frac\{1\}\{\backslash beta\}\; \backslash frac\{\backslash partial\}\{\backslash partial\backslash lambda\}\; \backslash ln\; Z(\backslash beta,\backslash lambda).$ 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.

Relation to thermodynamic variables
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 : $\backslash langle\; E\; \backslash rangle\; =\; -\; \backslash frac\{\backslash partial\; \backslash ln\; Z\}\{\backslash partial\; \backslash beta\}.$ The variance in the energy (or "energy fluctuation") is : $\backslash langle\; \backslash delta\; E^2\; \backslash rangle\; \backslash equiv\; \backslash langle\; (E\; -\; \backslash langle\; E\backslash rangle)^2\; \backslash rangle\; =\; \backslash frac\{\backslash partial^2\; \backslash ln\; Z\}\{\backslash partial\; \backslash beta^2\}.$ The heat capacity is : $C\_v\; =\; \backslash frac\{\backslash partial\; \backslash langle\; E\backslash rangle\}\{\backslash partial\; T\}\; =\; \backslash frac\{1\}\{k\_B\; T^2\}\; \backslash langle\; \backslash delta\; E^2\; \backslash rangle.$ The entropy is : $S\backslash equiv-k\_B\backslash sum\_j\; P\_j\backslash ln\; P\_j=\; k\_B\; (\backslash ln\; Z\; +\; \backslash beta\; \backslash langle\; E\backslash rangle)=\backslash frac\{\backslash partial\}\{\backslash partial\; T\}(k\_B\; T\; \backslash ln\; Z)\; =-\backslash frac\{\backslash partial\; F\}\{\backslash partial\; T\}$ where F is the free energy Helmholtz free energy defined as ''F = E - TS'', where ''E'' is the total energy and ''S'' is the entropy, so that : $F\; =\; \backslash langle\; E\backslash rangle\; -TS=-\; k\_B\; T\; \backslash ln\; Z.$

Partition functions of subsystems
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: : $Z\; =\backslash prod\_\{j=1\}^\{N\}\; \backslash zeta\_j.$ If the sub-systems have the same physical properties, then their partition functions are equal, ζ₁ = ζ₂ = ... = ζ, in which case : $Z\; =\; \backslash zeta^N.$ However, there is a well-known exception to this rule. If the sub-systems are actually identical particles, in the quantum mechanics quantum mechanical sense that they are impossible to distinguish even in principle, the total partition function must be divided by a ''N''! (''N'' factorial): : $Z\; =\; \backslash frac\{\backslash zeta^N\}\{N!\}.$ 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.

Examples
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.

Grand canonical partition function


Definition
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 $\backslash mathcal\{Z\}$ for an ideal quantum gas is written: : $\backslash mathcal\{Z\}\; =\; \backslash sum\_\{N=0\}^\backslash infty\backslash ,\backslash sum\_\{\backslash \{n\_i\backslash \}\}\backslash ,\backslash prod\_i\; e^\{-\backslash beta\; n\_i(\backslash epsilon\_i-\backslash mu)\}$ 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_''i''''n''_''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 : $\backslash prod\_i\; e^\{-\backslash beta\; n\_i(\backslash epsilon\_i-\backslash mu)\}=e^\{-\backslash beta(\backslash epsilon\_1-\backslash mu)\}\backslash ,e^\{-2\backslash beta(\backslash epsilon\_3-\backslash mu)\}.$ 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''.

Specific expressions
The above expression for the grand partition function can be shown to be mathematically equivalent to: : $\backslash mathcal\{Z\}\; =\; \backslash prod\_i\; \backslash mathcal\{Z\}\_i.$ (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: : $\backslash mathcal\{Z\}\_i\; =\; \backslash sum\_\{n\_i=0\}^\backslash infty\; e^\{-\backslash beta\; n\_i(\backslash epsilon\_i-\backslash mu)\}\; =\; \backslash frac\{1\}\{1-e^\{-\backslash beta\; (\backslash epsilon\_i-\backslash mu)\}\}$ and for a system composed of fermions: : $\backslash mathcal\{Z\}\_i\; =\; \backslash sum\_\{n\_i=0\}^1\; e^\{-\backslash beta\; n\_i(\backslash epsilon\_i-\backslash mu)\}\; =\; \backslash left(1+e^\{-\backslash beta\; (\backslash epsilon\_i-\backslash mu)\}\backslash right).$ For the case of a Maxwell-Boltzmann gas, we must use "correct Boltzmann counting" and divide the Boltzmann factor $e^\{-\backslash beta\; (\backslash epsilon\_i-\backslash mu)\}$ by ''n''_''i''!. : $\backslash mathcal\{Z\}\_i\; =\; \backslash sum\_\{n\_i=0\}^\backslash infty\; \backslash frac\{e^\{-\backslash beta\; n\_i(\backslash epsilon\_i-\backslash mu)\}\}\{n\_i!\}\; =\; \backslash exp\; \backslash left(\; e^\{-\backslash beta\; (\backslash epsilon\_i-\backslash mu)\}\backslash right).$

Relation to thermodynamic variables
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: : $\backslash langle\; n\_i\backslash rangle\; =\; -\backslash left(\backslash frac\{\backslash partial\backslash ln(\backslash mathcal\{Z\}\_i)\}\{\backslash partial\; \backslash alpha\}\; \backslash right)\_\{\backslash beta,V\}\; =\; \backslash frac\{1\}\{\backslash beta\}\backslash left(\backslash frac\{\backslash partial\backslash ln(\backslash mathcal\{Z\}\_i)\}\{\backslash partial\; \backslash mu\}\; \backslash right)\_\{\backslash beta,V\}.$ For Boltzmann particles this yields: : $\backslash langle\; n\_i\backslash rangle\; =\; e^\{-(\backslash beta\backslash epsilon\_i-\backslash mu)\}.$ For bosons: : $\backslash langle\; n\_i\backslash rangle\; =\; \backslash frac\{1\}\{e^\{\backslash beta(\backslash epsilon\_i-\backslash mu)\}-1\}.$ For fermions: : $\backslash langle\; n\_i\backslash rangle\; =\; \backslash frac\{1\}\{e^\{\backslash beta(\backslash epsilon\_i-\backslash mu)\}+1\}.$ 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 : $\backslash langle\; N\backslash rangle\; =\; -\backslash left(\backslash frac\{\backslash partial\backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; \backslash alpha\}\; \backslash right)\_\{\backslash beta,V\}\; =\; \backslash frac\{1\}\{\backslash beta\}\backslash left(\backslash frac\{\backslash partial\backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; \backslash mu\}\; \backslash right)\_\{\backslash beta,V\}.$
• Variance in total number of particles : $\backslash textrm\{var\}(N)\; =\; \backslash left(\backslash frac\{\backslash partial^2\backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; \backslash alpha^2\}\; \backslash right)\_\{\backslash beta,V\}.$
• Internal energy : $\backslash langle\; E\backslash rangle\; =\; -\backslash left(\backslash frac\{\backslash partial\; \backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; \backslash beta\}\; \backslash right)\_\{\backslash mu,V\}\; +\; \backslash mu\; \backslash langle\; N\backslash rangle.$
• Variance in internal energy : $\backslash textrm\{var\}(E)\; =\; \backslash left(\backslash frac\{\backslash partial^2\; \backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; \backslash beta^2\}\; \backslash right)\_\{\backslash mu,V\}.$
• Pressure : $\backslash langle\; P\backslash rangle=\backslash frac\{1\}\{\backslash beta\}\backslash left(\backslash frac\{\backslash partial\backslash ln(\backslash mathcal\{Z\})\}\{\backslash partial\; V\}\; \backslash right)\_\{\backslash mu,\backslash beta\}.$
• Mechanical equation of state : $\backslash langle\; PV\backslash rangle=\backslash frac\{\backslash ln(\backslash mathcal\{Z\})\}\{\backslash beta\}.$

Discussion
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.

References
* Huang, Kerson, "Statistical Mechanics", John Wiley & Sons, New York, 1967. * A. Isihara, "Statistical Physics", Academic Press, New York, 1971. * Kelly, James J, [http://www.physics.umd.edu/courses/Phys603/kelly/Notes/IdealQuantumGases.pdf (Lecture notes)] * L. D. Landau and E. M. Lifshitz, "Statistical Physics, 3rd Edition Part 1", Butterworth-Heinemann, Oxford, 1996. Category:Statistical mechanics de:Zustandssumme fr:Fonction de partition ru:СтатиÑ?тичеÑ?каÑ? Ñ?умма sl:StatistiÄ?na vsota

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