Test volume method: Difference between revisions

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An alternative to the [[virial pressure]] route to calculating the pressure, there is a method which consists on
The '''test volume method''' provides an alternative to the [[virial pressure]] route for calculating the [[pressure]] of a system. This method consists of
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>.
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>.


This should be tough of as a "blow-up" (or "in") of the whole system, <math>x \rightarrow (1+\alpha) x</math> with a small <math>\alpha</math> (same for <nowiki>y</nowiki> and <math>z</math>), exactly in the way as the [[virial pressure]] expression may be derived - not as leaving the particles in their places and changing the simulation cell (a procedure which does not make sense in general).
This can be thought  of as a "blow-up" (or "in") of the whole system, <math>x \rightarrow (1+\alpha) x</math> with a small <math>\alpha</math> (similarly for <math>y</math> and <math>z</math>), exactly in the same way as the [[virial pressure]] expression may be derived, i.e.  not as leaving the particles in their places and changing the simulation cell (a procedure which does not make sense in general).


It can be shown that
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>
:<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]].
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]].
The method is clearly inspired by the [[ Widom test-particle method]], which is used to obtain the [[chemical potential]].
 
It has been argued that this method can be more useful than [[virial pressure]], since it avoids the calculation of forces (which are not needed in a [[Monte Carlo]] simulation). Moreover, there are interaction potentials that are [[Idealised models#Piecewise continuous models |discontinuous]]. In such cases the forces are not well defined, and one would have to  consider the limit of "almost-discontinuous" potentials instead, which can be cumbersome. However, it should be noted that it is not clear how to "blow up" a system containing molecules held by rigid constraints.
It has been argued that this method can be more useful than [[virial pressure]] since it avoids the calculation of forces (which are not needed in a [[Monte Carlo]] simulation). Moreover, there are interaction potentials that are discontinuous - in this case, the forces are not well defined, and one should consider the limit of "almost-discontinuous" potentials instead, which can be cumbersome. However, it is not clear how to "blow up" a system containing molecules held by rigid constraints.
==References==
==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''' pp. 8469- (1996)]
#[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''' pp. 8469- (1996)]
[[category: computer simulation techniques]]

Revision as of 11:44, 7 February 2008

The test volume method provides an alternative to the virial pressure route for calculating the pressure of a system. This method consists of evaluating the change in internal energy, produced by a small change in the volume of the system, .

This can be thought of as a "blow-up" (or "in") of the whole system, with a small (similarly for and ), exactly in the same way as the virial pressure expression may be derived, i.e. not as leaving the particles in their places and changing the simulation cell (a procedure which does not make sense in general).

It can be shown that

where is the Boltzmann constant and is the temperature. The method is clearly inspired by the Widom test-particle method, which is used to obtain the chemical potential. It has been argued that this method can be more useful than virial pressure, since it avoids the calculation of forces (which are not needed in a Monte Carlo simulation). Moreover, there are interaction potentials that are discontinuous. In such cases the forces are not well defined, and one would have to consider the limit of "almost-discontinuous" potentials instead, which can be cumbersome. However, it should be noted that it is not clear how to "blow up" a system containing molecules held by rigid constraints.

References

  1. V. I. Harismiadis, J. Vorholz, and A. Z. Panagiotopoulos "Efficient pressure estimation in molecular simulations without evaluating the virial", Journal of Chemical Physics 105 pp. 8469- (1996)