Fine q-grid convergence

[Nandan]

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.

[Nandan]

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.

[sponce]

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

[Nandan]

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.

[sponce]

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

[Nandan]

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.

[carla.verdi]

Dear Nandan,

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

Best,
Carla

[Nandan]

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.

[Nandan]

Carla,

Sorry for spelling your name wrong earlier.

nandan.

[Nandan]

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.