Editing Test volume method

An alternative to the [[virial pressure]] route to calculating the pressure, there is a method which consists on
evaluating the change in [[internal energy]], <math>\Delta U</math> produced by a small change in the volume of the system <math>\Delta V</math>. It can be shown that

:<math> p  =  \frac{ k_B T  N}{V} + \frac{ k_B T  }{ \Delta V } \log \langle \exp(-\Delta U/k_B T )\rangle,</math>
where <math>k_B</math> is the [[Boltzmann constant]] and <math>T</math> is the [[temperature]].

The method is clearly inspired by the [[ Widom test-particle method]] to obtain the [[chemical potential]].

==References==
#[http://dx.doi.org/10.1063/1.472721   V. I. Harismiadis, J. Vorholz, and A. Z. Panagiotopoulos "Efficient pressure estimation in molecular simulations without evaluating the virial", Journal of Chemical Physics '''105''' p. 8469  ]