[Pw_forum] Free energy and entropy

[Axel Kohlmeyer]

On Sun, Jun 23, 2013 at 11:58 AM, Alex Sham <as.sham.okc at gmail.com> wrote:
> Dear QE users,
>
> I'm trying to use vc-relax to determine the optimal structure of an alloy at
> room temperature & room pressure. To do that I will need to compare the free
> energy of the system with various structures, which needs the entropy of the
> relaxed system. Does anyone know if QE provides such a calculation. (I know
> that the calculation of entropy may be very difficult...)  Or if not, does
> one know if vc-relax can relax the system in a canonical ensemble under a
> certain external temperature? Or if not, any methods that can achieve the
> similar end?

dear alex,

as you already noticed, once you move to a finite temperature, things
get messy. if you want to compute the entropy component of the free
energy surface, you need to find out how probable a given
configuration is, which requires to know how many configurations of
the same energy there are. give then large number of degrees of
freedom, that is impossible, but you may be able to get a good
approximation, especially, if you have some existing knowledge about
what possible transformations can happen and thus reduce the numbers
of degrees of freedom into a few "collective variables".

with those you may be able to do umbrella sampling or metadynamics in
combination with a molecular dynamics run. there may also be some more
approximative methods (you will need to search the literature), since
MD can be quite demanding and converging umbrella sampling or
metadynamics runs to a sufficient level of accuracy might need a
significant chunk of it.

HTH,
     axel.

 you essentially have to map out the free energy landscape and look
for minima in them.

>
> Thank you very much for your time and help,
> Alex Sham
>
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--
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.