[Pw_forum] Phase Transition Pressures

[Stefano Baroni]

there are no other relevant contributions to the entropy than phonons and (to a much lesser extent) electrons. my only point was that they are both irrelevant in the low-temperature regime which I presume cecil is mostly interested in. what is instead far from being irrelevant is the difference between different thermodynamical potential, which may be an obvious concept to an experimented researcher, not necessarily so to a student. the important thing to remember is that at finite pressuer the most stable structure is that which minimizes the enthalpy (or Gibbs free energy), and NOT the energy (or Helmholtz free energy). Hope this helps. Stefano

On Apr 18, 2010, at 3:35 PM, Huolin Xin wrote:

> Stefano, thanks a lot for pointing my mistake out. In the entropy term, besides phonon and electronic entropy, how to calculate the structural entropy?
> 
> On Sun, Apr 18, 2010 at 9:25 AM, Stefano Baroni <baroni at sissa.it> wrote:
> Dear Huolin: as explained in my previous post, I beg to differ. Phonons have nothing to do with the question asked by Cecil, as long as the temperature is low enough. Even less, the electronic entropy, as long as the the system insulating (even in metals, the electronic entropy play a very little role, below the Farmi temperature which is typically several thousands degrees ...). Even considering temperature effects, it is not the "free energy" that has to be made equal (usually, one indicates with this term the Helmholts free energy, E-TS), but the "Gibbs free energy" (E-TS+PV). Hope this clarifies a bit ... SB
> 
> On Apr 18, 2010, at 3:04 PM, Huolin Xin wrote:
> 
>> You'll need phonon entropy and electronic entropy as well. Free energy is E-TS. The transition pressure is where the free energy of the two structures are the same. 
>> 
>> On Sun, Apr 18, 2010 at 5:39 AM, naphtaly moro <moronaphtaly84 at gmail.com> 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?
>> 
>> 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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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

Please, if possible, don't  send me MS Word or PowerPoint attachments
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