[Pw_forum] imaginary phonon frequency for different degauss

[Miao Gao]

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

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Ph.D. student of Department of Physics,
Renmin University of China.
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