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Fine q-grid convergence 
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Fine q-grid convergence
Hi,

For GaAs I have used two fine q-mesh grids: 1. 50x50x50 and 2. 80x80x80
keeping the fine k-grid the same (a path along L-G-X with 25000 points). But I get two different
results for Im(Sigma) as shown in the link below. They look similar but the magnitudes differ significantly.

http://www.mediafire.com/?f22v94bes7rwa

All other details of the calculation are identical including the machine on which the job was run.

Any idea why this is happening?

Thanks and regards,

Nandan.


Fri Feb 16, 2018 5:28 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
If it helps, I am using the following input file:

--
&inputepw
prefix = 'gaas'
amass(1) = 69.72300
amass(2) = 74.92160
outdir = './'

! ephwrite = .true.

elph = .true.
kmaps = .true.
epbwrite = .false.
epbread = .false.
epwwrite = .false.
epwread = .true.

etf_mem = 0

nbndsub = 12
nbndskip = 0

dis_win_max = 29
dis_froz_max= 14
dis_froz_min= -8

restart = .true.
restart_freq= 2000

wannierize = .false.
lpolar = .true.
num_iter = 5000
iprint = 2
proj(1) = 'random'
proj(2) = 'As:sp3'

! efermi_read = .false.
! fermi_energy= 5.4304

elecselfen = .true.
phonselfen = .false.

band_plot = .true.
parallel_k = .true.
parallel_q = .false.

iverbosity = 3
fsthick = 25
eptemp = 300.d0
degaussw = 0.005

dvscf_dir = '../phonons/save'
filukk = './gaas.ukk'
filkf = './LGX-25K.pwscf'

nk1 = 6
nk2 = 6
nk3 = 6

nq1 = 6
nq2 = 6
nq3 = 6

! nkf1 = 20
! nkf2 = 20
! nkf3 = 20
!
nqf1 = 50
nqf2 = 50
nqf3 = 50

! rand_k = .true.
! rand_nk = 8000

!mp_mesh_k = .true.
! rand_q = .true.
! rand_nq = 125000

/
16 cartesian
0.0000000 0.0000000 0.0000000 0.0092593
-0.1666667 0.1666667 -0.1666667 0.0740741
-0.3333333 0.3333333 -0.3333333 0.0740741
0.5000000 -0.5000000 0.5000000 0.0370370
0.0000000 0.3333333 0.0000000 0.0555556
-0.1666667 0.5000000 -0.1666667 0.2222222
0.6666667 -0.3333333 0.6666667 0.2222222
0.5000000 -0.1666667 0.5000000 0.2222222
0.3333333 0.0000000 0.3333333 0.1111111
0.0000000 0.6666667 0.0000000 0.0555556
0.8333333 -0.1666667 0.8333333 0.2222222
0.6666667 -0.0000000 0.6666667 0.1111111
0.0000000 -1.0000000 0.0000000 0.0277778
0.6666667 -0.3333333 1.0000000 0.2222222
0.5000000 -0.1666667 0.8333333 0.2222222
-0.3333333 -1.0000000 0.0000000 0.1111111

Nandan.


Wed Feb 21, 2018 8:34 pm
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Joined: Wed Jan 13, 2016 7:25 pm
Posts: 573
University: Oxford
Post Re: Fine q-grid convergence
Hello Nandan,

I do not see any obvious issue.

Do you really need 25,000 points along two lines ?

What I would do is to reduce the number of k-point to 250 instead and then do a few q-grids (homogeneous and random).

Note that in polar materials it will converge slowly with the q-grids (you probably need 300x300x300 or so).
You should use random q-grid or better some grids that over-sample the divergence in Gamma (for example a log grid).
Make sure that the weights are correct.

Best,
Samuel

_________________
Dr. Samuel Poncé
Department of Materials
University of Oxford
Parks Road
Oxford OX1 3PH, UK


Wed Feb 21, 2018 10:28 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
Hello Samuel,

Thanks for the reply.

I had not considered the weights of the k-points at all. I have been using constant weights for
the k-points in the list. For N-kpoints, the weight for each is 1/N.
How do I create a k-list with correct weights?

Nandan.


Thu Feb 22, 2018 4:42 pm
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Joined: Wed Jan 13, 2016 7:25 pm
Posts: 573
University: Oxford
Post Re: Fine q-grid convergence
If you are interested in linewidths (so values for a given k-point) it does not matter.

However the weight for the q-points are important since you do a q-point integration.
The weight of the q-points should be equal to their volume in the BZ.

For a log grid, you will have much more points close to gamma and therefore their weight should be smaller.

Best,
Samuel

_________________
Dr. Samuel Poncé
Department of Materials
University of Oxford
Parks Road
Oxford OX1 3PH, UK


Fri Feb 23, 2018 2:53 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
Hi Samuel,

So after doing another calculation with a denser fine q-grid of 10^6 points and a coarse k-grid of 250 (red) points (or 5000:green) along L-G-X, I get the
scattering rates (in terms of Im(Sigma) meV) as shown below:

http://www.mediafire.com/file/l70neisng ... -1mill.eps

If this is compared with Phys. Rev. B 94, 201201(R) (2016), Fig 1, then the magnitudes of Im(sigma) are atleast an order higher
in my calculation.

Where as for a coarse grid as I had done earlier in http://www.mediafire.com/file/9173kpgq2 ... h-comp.eps,
the magnitude is comparable to the reference.

I am trying to understand why is the magnitude so different in the two cases compared to the above reference?

Nandan.


Tue Feb 27, 2018 4:06 pm
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Joined: Thu Jan 14, 2016 10:52 am
Posts: 150
University: University of Vienna
Post Re: Fine q-grid convergence
Dear Nandan,

Have you checked that the interpolated band structure and phonons that you get from epw are correct?

Best,
Carla


Tue Feb 27, 2018 4:31 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
Hi Cara,

The interpolated electrons and phonons look fine. There is a small negative frequency near Gamma,
but overall it looks alright.
phonons: http://www.mediafire.com/file/hxye197xq ... phonon.eps
electrons: http://www.mediafire.com/file/9rihxg4mw ... ctrons.eps

Nandan.


Tue Feb 27, 2018 8:43 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
Carla,

Sorry for spelling your name wrong earlier.

nandan.


Tue Feb 27, 2018 8:45 pm
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Joined: Mon May 09, 2016 2:47 pm
Posts: 42
Post Re: Fine q-grid convergence
I am wondering if this could be compiler related issue?
Or do the negative phonon frequencies near Gamma cause this behavior in Im(sigma)?

Thanks and regards,

Nandan.


Tue Mar 06, 2018 9:34 pm
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