[Pw_forum] Lattice constant for copper

[Nicola Marzari]

On 3/3/11 12:32 PM, Eric Germaneau wrote:
> Hey,
>
> I'm a newbie too and I'm wondering how can you equilibrate the lattice
> constant by doing just a scf calculation.
> Also, it seems to me that only MD is able to reproduce experimental data
> since it takes into account the temperature and the pressure.
> Please, let me know if I'm wrong.
>
> Eric.
>


Dear Eric, All,


just a brief comment re temperature - vibrational excitations follow
Bose-Einstein statistics (nuclei are indistinguishable) rather than
Maxwell-Boltzmann. Molecular dyanmics, though, will give you a
Maxwell-Boltzmann distribution - these two differ,
especially at temperatures below the Debye temperature.

The more appropriate thing to do to calculate thermal expansion is
to construct the free energy of the system adding the vibrational
entropy, and minimize the free energy that at any given temperature as
a function of celldm(1:6). Plenty of literature on the topic.

			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