[Pw_forum] imaginary phonon frequency for different degauss

[Duy Le]

Without info. about smearing method, it is hard to tell. Some methods
may give you incorrect forces, resulting in incorrect phonon.
degauss and k-point mesh should be well tested TOGETHER.

Check out the following:
http://caribeictp.uis.edu.co/lectures/R.Gebauer/Gebauer_Course2_KpointsMetals.pdf

----------------------------------------------------
Duy Le
Postdoctoral Associate
Department of Physics
University of Central Florida.
Website: http://www.physics.ucf.edu/~dle


On Sat, Jun 22, 2013 at 9:05 AM, Miao Gao <miaogao at ruc.edu.cn> wrote:
> Dear all：
>
> I am calculating phonon with QE-5.0.1.
> A 20x20x10 k-mesh is used in scf calculation. Phonon wave vectors are
> sampled
> on 10x10x5 mesh. But when I set degauss=0.03Ry in scf run, a imaginary
> frequency
> was found for the first branch with omega= -105.6 cm^-1 at wave vector (0.2,
> 0.346410162, 0.0).
> But if degauss=0.05Ry in scf run, this imaginary frequency disappear and
> become 132.1 cm^-1.
> Meanwhile, the frequencies of all other branches change slightly for these
> two degauss values.
> So my question is which results should I trust, degauss=0.03 Ry or 0.05Ry?
>
> Btw: both lattice parameters and the inner coordinates of atoms are well
> relaxed for
> different degauss values.
>
> I am greatly indebted to you for your help. Thanks again!
>
> Best
> Miao Gao
>
> ---------------------------------------
> Ph.D. student of Department of Physics,
> Renmin University of China.
> ---------------------------------------
>
>
>
>
>
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