[Pw_forum] Convergence or band-gap problem in orthorhombic CsPbBr3

Christoph Wolf(신소재공학과) chwolf at postech.ac.kr
Thu Sep 8 14:55:20 CEST 2016

Dear Prasenjit!

Thank you for your suggestion, this paper was, actually, my starting point. You have used US PP but for YAMBO I have to use normcons, otherwise my approach (vdW, …) was heavily “inspired” by your suggested paper.

I find it highly interesting that the gap sometimes opens at Gamma (your work in the supporting info) and sometimes at other points in the BZ. Could you briefly comment if you started relaxation from the ideal orthorhombic cell and vc-relax and  relax the system (at gamma point or the whole k-grid given as 8x6x8)?  Did you run it with a minimal unit cell (5 atoms cell) or a larger cell to begin with? You obtain a ~3% volume difference with vdW, for the standard PBE normcons PP I actually obtained up to 30% difference, where the cell shrinks drastically and “elongates” along the longest axis. Probably my PP are not a good choice but yambo somewhat limits my choice.

Dear Frederico!

Thank you for your suggestions. I just started yambo a few weeks ago and I am sure there are additional problems with my yambo inputs (which will appear in the yambo board…).  You are probably right, I might have to converge the k-grid further (so far I have only considered wrt the DFT total energy which has given a good compromise between 4x and 6x in terms of convergence and computational time but maybe 4x3x4 is just too coarse).

I considered nbnd=200 bands and yambo tells me that up to the 17th band the system is occupied which gives me plenty of empty bands (hopefully enough).

I will be back with the results when the calculation is complete. Thank you both very much for your help!


Christoph Wolf

Department of Materials Science and Engineering, POSTECH
RIST #3, 3152, San 31 Hyoja-dong, Nam-gu, Pohang, Kyungbuk, 790-784, Republic of Korea

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