[Pw_forum] J.Breeze on:Third order anharmonic dynamical matrix

[Michele Lazzeri]

>Using the PWSCF D3 code, should it be possible to calculate the
>coefficients for phonons at arbitraty wave-vector?
>That is, to calculate D^3(-q1-q2,q1,q2).

>Jonathan Breeze

Dear Jonathan,
As you know the code calculates D^3(0,-q, q).
More in general, to compute D^3(-q1-q2,q1,q2) is possible, but the
modification to be done to the code could be tedious.

In alternative, you could use a super-cell approach:
If you want to compute D^3(Q,Q-q, Q+q), you can compute
D^3(0,-q,q) in a super-cell such that the gamma-point of the
super-cell is equivalent to the Q point of the unit-cell.
An example: if you want to compute the broadening of a phonon
at the border zone, you can use a crystal obtained doubling the
unit-cell
in the appropriate direction.
The phonon you are interested in is a phonon at Gamma for this
double-cell.
So, if you compute the D3 matrix at gamma, for this double-cell, you
obtain
all the information that you need.

For the moment, this is the most practical thing you can do if you are
interested in high-symmetry points.
This procedure cannot be applied to any point Q of the Brillouin Zone,
and, depending on the point, can be computationally expensive.

Michele Lazzeri



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20030919/35df39d4/attachment.html>