[Pw_forum] help: Pt phonon calculation
Stefano Baroni
baroni at sissa.it
Sun Sep 17 17:26:54 CEST 2006
On Sep 17, 2006, at 3:13 PM, hailin yu wrote:
> Dear all:
> I have some questions about the phonon calculation of Pt.
> 1). Is it reasonable to calculate the phonon dispersion of Pt
> by using the Pt.pbe-rrkjus.UPF pseudopotentials?
why not?
> 2). For the value of degauss, how it is reasonable? And for
> the kpoint?
Only you can tell. This is not so difficult to assess. Take a few
representative points in the BZ and calculate the phonon frequencies
as functions of degauss. For each value of degauss, keep increasing
the number of k points until you reach convergence. Note: (1) the
resulting number of k-pts should increase with decreasing degauss;
(2) this is the order to follow (i.e. degauss first, convergence wrt
to k-points for each given value of degauss: not the other way
around). Finally choose the value of degauss (and the corresponding
number of k-pts) which gives you the accuracy you desire.
In normal circumstances, such as Pt, I would say, the optimal value
of degauss should not depend on the phonon wavevector. At a Kohn
anomaly, instead, a much smaller degauss (and a correspondingly
larger number of k-pts) than at "ordinary" phonon wave-vectors is
usually needed. Can you see why? (if you understand this you
understand a lot of the fine details of phonon calculations in metals).
Take care
Stefano
---
Stefano Baroni - SISSA & DEMOCRITOS National Simulation Center -
Trieste
[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
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