[Pw_forum] gibbs free energy

[Nicola Marzari]

On 2/5/11 3:04 PM, Eric Germaneau wrote:
> Dear all,
>
> I'm new in using DFT codes and especially pwscf.
> I'd like to calculate the Gibbs free energy of a crystal as function of
> temperature in order to study it's stability.
> How would you proceed?
> I thank you in advance,
>
> Eric.
>


Hi Eric,

Gibbs is E+PV-TS - the enthalpy term is easy (pwscf deals e.g.
with constant pressure calculations, or you could just calculate the
equation of state and from that the enthalpy), and for the entropy
the most relevant part is the vibrational entropy, that you could
e.g. calculate in the harmonic or quasiharmonic approximations.

Have a look at e.g. the papers of Dario Alfe and Mike Gillan
(~1998-2005) - some of them deal with the more complex problem of 
calculating the free energies of liquid and solid iron, but I
believe there was one dealing just with solid-solid transformations.

A brief tutorial on quasiharmonic is also in our PRB 71, 205214 (2005)

Best,

			nicola

-- 
----------------------------------------------------------------------
Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM