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

[mkondrin]

Hello!

I have tried both suggestions-- increased the convergence threshold to 
1e-8 and used the converged values of CELL_PARAMETERS (so in the phonon 
calculation it was less than experimental one:
CELL_PARAMETERS (alat= 10.69207046)
   -0.496242253  -0.000000000   0.496242253
    0.000000000   0.496242253   0.496242253
   -0.496242253   0.496242253  -0.000000000
)

The result is better (\omega_max=298 cm^-1) but still is not perfect 
(thick magenta curve in the attached figure). It seems that the 
germanium have to be squeezed further to fit experimental DOS.

Sincerely yours,
Mikhail Kondrin

On 24.12.2020 12:39, mkondrin wrote:
> Hi, Paolo!
>
> Thank you for the hints. I will try all of them.
>
> Sincerely yours,
> Mikhail Kondrin
>
> On 23.12.2020 20:09, Paolo Giannozzi wrote:
>> 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
>>
>>
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