Virial equation of state

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The virial equation of state is used to describe the behavior of diluted gases. It is usually written as an expansion of the compressibility factor, , in terms of either the density or the pressure. Such an expansion was first introduced in 1885 by Thiesen [1] and extensively studied by Heike Kamerlingh Onnes [2] [3], and mathematically by Ursell [4]. One has



  • is the pressure
  • is the volume
  • is the number of molecules
  • is the temperature
  • is the Boltzmann constant
  • is the (number) density
  • is called the k-th virial coefficient

Virial coefficients[edit]

The second virial coefficient represents the initial departure from ideal-gas behaviour

where is Avogadros number and and are volume elements of two different molecules in configuration space.

One can write the third virial coefficient as

where f is the Mayer f-function (see also: Cluster integrals). See also [5]


For a commentary on the convergence of the virial equation of state see [6] and section 3 of [7].

Quantum virial coefficients[edit]

Using the path integral formulation one can also calculate the virial coefficients of quantum systems [8].


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