[Wien] Phase-Transition.

[Peter Blaha]

> You can use the Phonon package, available from K.Parlinski, with the Wien
> code to calculate the phonon spectrum and from this calculate the lattice
> entropy contribution to the free energy.  Does that answer your question?
> There are other codes, however, which use a perturbative method, rather
> than direct calculation of the force constants, to calculate the phonon
> dispersion which may be more suitable (i.e. faster) for your needs but
> this is not available in Wien2k.
>
> A question to the developers: Are there any plans to implement linear
> response in Wien2k?

linear response has been implemented by C.Ambrosch-Draxl. However, as far
as I know from her, the status of that program is not in a way that it
can be distributed. Eventually you may contact her directly.

For the question of phase-transitions I doubt that linear response is
much faster anyway. (Most linear respons programs are based on pseudo-
potentials and do not work well for Transition metals)

In order to calculate these thermodynamic functions you need to integrate
the phonons over the Brilloinzone and as far as I know you need a very good
q-mesh (q is the phonon-wavevektor) for this. That means with linear response
you would need a seperate calculation for each q (and you need many of them).
This is probably still faster for elemnts or binaries,... but when your
unitcell is large anyway, the linear response calc. take also very long.

Using PHONON one calculates first generalized force constants (using forces
from WIEN2k; i.e. you need a couple of supercell calculations (or the
regular unit cell, if this is already a large cell) of high
precision), but once you have this, you can calculate the phonons at any
q, thus it can do quite accurate integrals over q and gives good
thermodynamics.

Regards

                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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