[QE-users] I have failed to reproduce phonon DOS of germanium

[Paolo Giannozzi]

On Wed, Dec 23, 2020 at 1:46 PM mkondrin <mkondrin at hppi.troitsk.ru> wrote:

&SYSTEM
>    ibrav = 0
>    A =   5.658
>

A is the experimental lattice parameter and would be fine for ibrav=2
(fcc), but then you set your lattice vectors:

CELL_PARAMETERS (alat= 10.69207046)
>   -0.510237966   0.000000000   0.510237966
>    -0.000000000   0.510237966   0.510237966
>    -0.510237966   0.510237966   0.000000000
>

to a larger cell (they are in "alat" unit, that is, the value specified as
"A" or "celldm(1)", NOT the value in the card). With ibrav=2 you get
something like 290 cm^-1. Note that the phonons are typically computed at
the theoretical equilibrium lattice parameter, that in LDA is typically
smaller than the experimental, leading to higher frequencies.

Also note that the convergence threshold conv_thr=1.e-5 in the scf
calculation is too large: reduce it to at least 1.0e-8 (it costs close to
nothing with respect to the phonon calculation)

Paolo
-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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