[Pw_forum] EPC of Al at X
Amit Kumar
amit76.india at gmail.com
Fri Mar 30 21:05:04 CEST 2007
>>>>>>>>
would you mind reading the answer that you get before
asking new questions, or more exactly, the same question?
Dear Paolo,
I read your every answer very carefully.
My question was not the same.
I agree that Methfessel-Paxton or Gaussian broadening might change the
absolute
value of Lambda little bit.
However,
last time I wanted to say that electron-phonon matrix elements are not
converged using 32x32x32 k-point grid. I know achieving convergency is
very
slow and painful. But I was very surprised to see the value of averaged
'lambda' in the
example 's out put directrory. How come it's so close to the
experimental value
even for very low el-ph broadening (0.01 Ryd).
Please check it once.
Electron-phonon coupling constant, lambda
Broadening 0.0100 lambda 0.3845 dos_el 1.8818
Broadening 0.0200 lambda 0.3744 dos_el 2.2498
Broadening 0.0300 lambda 0.3447 dos_el 2.3960
Broadening 0.0400 lambda 0.3446 dos_el 2.5079
Broadening 0.0500 lambda 0.3515 dos_el 2.5896
Broadening 0.0600 lambda 0.3555 dos_el 2.6381
Broadening 0.0700 lambda 0.3551 dos_el 2.6616
Broadening 0.0800 lambda 0.3516 dos_el 2.6709
Broadening 0.0900 lambda 0.3463 dos_el 2.6737
Broadening 0.1000 lambda 0.3405 dos_el 2.6743
When individual matrix elements are not well converged (Please see my last
mail)
then how come lambda value is so good even for small broadening and
small nq value (4 4 4)???? Many things confuse me lot -----
Which dos_el or Fermi energy or double delta integral are
acceptable???
Sometime with increasing k-point grid we did not get better convergence
---
it seems the value changes very slowly with increasing K-point.
We can't take arbitrary large k-point grid (millions of K-points) because
it demands large
disk space and very long computational time.
Do you think the following numbers are converged???????
. Gaussian Broadening: 0.010 Ry, ngauss= 0
DOS = 1.881758 states/spin/Ry/Unit Cell at Ef= 8.327154 eV
lambda( 1)= 0.0253 gamma= 0.92 GHz
lambda( 2)= 0.0291 gamma= 1.05 GHz
lambda( 3)= 0.0403 gamma= 6.35 GHz
Gaussian Broadening: 0.020 Ry, ngauss= 0
DOS = 2.249756 states/spin/Ry/Unit Cell at Ef= 8.324326 eV
lambda( 1)= 0.0699 gamma= 3.02 GHz
lambda( 2)= 0.0781 gamma= 3.37 GHz
lambda( 3)= 0.1272 gamma= 24.01 GHz
Gaussian Broadening: 0.030 Ry, ngauss= 0
DOS = 2.396042 states/spin/Ry/Unit Cell at Ef= 8.311302 eV
lambda( 1)= 0.0799 gamma= 3.67 GHz
lambda( 2)= 0.0856 gamma= 3.93 GHz
lambda( 3)= 0.1515 gamma= 30.44 GHz
Gaussian Broadening: 0.040 Ry, ngauss= 0
DOS = 2.507879 states/spin/Ry/Unit Cell at Ef= 8.299961 eV
lambda( 1)= 0.0851 gamma= 4.10 GHz
lambda( 2)= 0.0885 gamma= 4.26 GHz
lambda( 3)= 0.1599 gamma= 33.63 GHz
Gaussian Broadening: 0.050 Ry, ngauss= 0
DOS = 2.589584 states/spin/Ry/Unit Cell at Ef= 8.291558 eV
lambda( 1)= 0.0881 gamma= 4.38 GHz
lambda( 2)= 0.0901 gamma= 4.48 GHz
lambda( 3)= 0.1645 gamma= 35.73 GHz
Gaussian Broadening: 0.060 Ry, ngauss= 0
DOS = 2.638140 states/spin/Ry/Unit Cell at Ef= 8.285378 eV
lambda( 1)= 0.0887 gamma= 4.49 GHz
lambda( 2)= 0.0900 gamma= 4.56 GHz
lambda( 3)= 0.1673 gamma= 37.02 GHz
Gaussian Broadening: 0.070 Ry, ngauss= 0
DOS = 2.661607 states/spin/Ry/Unit Cell at Ef= 8.280404 eV
lambda( 1)= 0.0876 gamma= 4.47 GHz
lambda( 2)= 0.0883 gamma= 4.51 GHz
lambda( 3)= 0.1695 gamma= 37.82 GHz
Gaussian Broadening: 0.080 Ry, ngauss= 0
DOS = 2.670887 states/spin/Ry/Unit Cell at Ef= 8.275903 eV
lambda( 1)= 0.0856 gamma= 4.39 GHz
lambda( 2)= 0.0859 gamma= 4.40 GHz
lambda( 3)= 0.1717 gamma= 38.47 GHz
Gaussian Broadening: 0.090 Ry, ngauss= 0
DOS = 2.673746 states/spin/Ry/Unit Cell at Ef= 8.271433 eV
lambda( 1)= 0.0834 gamma= 4.28 GHz
lambda( 2)= 0.0834 gamma= 4.28 GHz
lambda( 3)= 0.1744 gamma= 39.10 GHz
Gaussian Broadening: 0.100 Ry, ngauss= 0
DOS = 2.674314 states/spin/Ry/Unit Cell at Ef= 8.266772 eV
lambda( 1)= 0.0813 gamma= 4.17 GHz
lambda( 2)= 0.0811 gamma= 4.16 GHz
lambda( 3)= 0.1773 gamma= 39.76 GHz
and so on ..........................
It keeps on incresaing forever even with a very large K-point grid.
Then how come averaged 'lamda' value is so closed to the experimental
value
even with small Gaussian broadennig and small K-point grids (like 16
16 16) ???????
Is it accidental??????
Should we take large value of nq like nq1=32, nq2=32, nq3=32
like large value of nk for better results?????????
Sometime even in total energy calculatiion we may get accidental
convergence.
In MIT lecture notes, it's written that
You do need to be careful though. It is possible to get "false" or
"accidental"
convergence as well. That is, your energy at a 2x2x2 k-grid may be the same
as
the energy at a 8x8x8 k-grid, but the energy at a 4x4x4 might be very
different
from both of these. In this case, you aren't really converged at a 2x2x2
k-grid.
Is it possible to calculate EPC for arbitrary q -point like
0.13579 0.3474 0.83765 ???????????
Looking forward to your valuable suggestions.
With best regards,
Amit
P.S. Dear Nicola, Thank you very much for your useful reference.
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