# [Pw_forum] Phase Transition Pressures

Stefano Baroni baroni at sissa.it
Sun Apr 18 15:18:53 CEST 2010

```On Apr 18, 2010, at 11:39 AM, naphtaly moro wrote:

> Dear all,
>
> I am working on the pressure induced phase transitions of GaAs, I have
> already obtained, the respective EV data for the structures and want
> to get the transition pressures for the structures, Any advice on how
> this can be done in Espresso?

As with any other code: using the Maxwell construction (if you have never heard about this concept, browse any book in classical thermodynamics / elementary statistical mechanics and/or google). In a nutshell: you plot the two E(V) curves for the two structures, and you look for a straight line that is tangent to both curves: the slope of the straight line is the transition pressure. If you want to understand why this construction works, remember that at any given pressure the stable structure is the one that minimizes the enthalpy of the system (E+PV) or the Giibbs free energy (E-TS+PV). In both these expressione remember that the volume is a *dependent* variable that has to be expressed in terms of the pressure (the independent variable). Start from this, and try to work out the Maxwell construction by yourself ...

Cecil: one thing that every student has to learn is how to grade different methods and concepts. In the present case, you will see that the concepts needed to solve your problem are rather general and have little (if anything) to do with the specific code you use to obtain the E(V) data. I am saying this not to suggest that you shouldn't have asked this question on this mailing list: you did well, and you are most welcome to ask similar questions in the future. Only, I would like to make sure that you see the difference between what is general and what is specific.

Good luck - Stefano B

>
> Thanks
> Cecil Naphtaly Moro,
> Mphil. Student,
> Department of Physics,
> Moi University, Chepkoilel Campus,
> Kenya
>
> --
> Cecil Naphtaly Moro
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

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