[Pw_forum] phonon dispersion with experimental structure

[Axel Kohlmeyer]

On Thu, May 2, 2013 at 10:11 AM, ding <dingmingcui at qq.com> wrote:
> Dear all and stefano:
>       Thanks for your reply. Do you mean there is no way to calculate the
> phonon dispersion of the same material at different pressures? As is known,
> the structures are
> ususlly different at different pressures.  But in priciple there is only one
> equilibrium  structure when pressure is not apllied. Can the pressure factor
> be considered when we do phonon calculation?

you have basically given the answer yourself already. this is at heart
a problem of statistics. in the 0K case the situation is simple,
because a minimum structure is a well defined entity and then it is
straightforward to compute phonons and similar properties based on a
harmonic approximation (which in turn can be approximated with
perturbation theory or finite differences).

for a finite temperature system the one (minimum) structure becomes an
ensemble of equivalent structures that you have to average over
according to their probabilities. that doesn't mean that it is
impossible to compute phonon dispersions, but you have to use a
different approach, and that may likely be too CPU time demanding to
be done with a DFT code. for example, there is a finite temperature
phonon dispersion facility as an optional package to the LAMMPS
classical MD code. you may want to check out the provided references.

http://lammps.sandia.gov/doc/fix_phonon.html

axel.

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--
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
International Centre for Theoretical Physics, Trieste. Italy.