el-ph coupling in larger supercells

[chelsy]

Dear all,

I am trying to understand if it is possible to reduce computational time for electron-phonon scattering rate calculations in larger super cells. I started with computation of el-ph scattering rate for silicon 2 atom u.c. using parameters from Computer Physics Commun, 209, 116, 2016. (Coarse grid: 6x6x6/6x6x6; fine grid(q/k): 150,000/30,000). I perfectly matched the scattering rate plot with one in the paper. Then, I increased the cell to 8 atoms (conventional cell). For this system I used the following parameters: coarse grid: 4x4x4/4x4x4; fine grid(q/k):30,000/10,000; for sp3 hybridization I get 32 wannier functions. Since I use 64 k-points for the coarse grid I can only use 64 cores for calculations (one k-point per processor). Thus, such calculations took about 3 days on HPC. If I keep increasing the size of the cell, the BZ is decreasing and, therefore, the number of k-points can be decreased as well. This means I would have to reduce the number of cores for calculations. I also tried to perform calculations for supercell containing 2 conventional cells (16 atoms) and it took almost 2 weeks on 32 cores. At this point I am not sure if I can further increase the cell size. I would really appreciate if anyone could suggest me how to reduce the computational time.

1. Is there any way to use more cores than number of k-points on coarse grid?
2. For sp3 hybridization in Si I can only specify 4 Wannier function per atom. Therefore, 16 atom u.c. would have 64 w.f. Can I reduce the number of wannier functions somehow?
3. I noticed that the frozen window size doesn't affect the results significantly. Can I use larger window size then?

Thank you very much!

[sponce]

Dear chelsy,

Happy to hear that you could reproduce the result of the paper.
First of all, why do you want to do supercells ? It should give you the same answer I guess (except if you do defects or something else ?)

The code works in two parts (i) the part before the writing of the el-ph on real space and (ii) the interpolated part. For the first part, you can indeed parallelize on k-points, which mean you will be able to parallelize on less if you reduce the coarse k-point grid.
However I implemented a band parallelism for that reason. It is not well tested (I used it for 1 system I was looking at) but it is inside the code but not well documented.
The way it works is by using image parallelization. This only works for part (i).
There is a test in q-e/test-suite/epw* about it.
Also, you should look into the code to really understand it.

For part (ii) it should not be a problem if you are using those fine grids.

Note however that the code is not really design to do supercell calculations so it is not really optimized for this.

PS: I'm going on holiday tomorrow morning for a week so I cannot help you more until then unfortunately.

[topgun]

can you please elaborate on supercell method and DFPT methods. i mean the difference between them? are calculations using supercell method more heavier than DFPT method? lastly, EPW have a feature for DFPT calculations and not supercell method.

topgun