[QE-developers] Bad scalability of supercells energy with Hybrid functional.

[jmasnaghetti]

Dear QE developer,

I'm a master student from the University of Milan and I've been working 
on simulating Silicon Germanium alloys with hybrid functional for my 
master thesis. I've already posted a similar question to the users 
forum, but I've been advised by my supervisors to post it to the 
developer as it may be related to something in the actual code of QE.
Basically, my problem seems to be the fact that, running some tests, the 
energies per ion are not consistent between the classic 2 ions cells and 
bigger 8 or 16 atoms supercells if i use ultra-soft pseudos. The 
differences are of the order of tens of meV. For comparison, I've always 
set my convergence parameters so that the energies are correct within 1 
meV, and that seems to be the case for norm conserving pseudos, form 
whom the per ion energy differences between the conventional cell and 
supercell are of the order of 0.1 meV. I did not change any of the 
energy cutoffs between the conventional cell and the supercells and i 
tried to keep the products between the number of k points and atoms 
consistent across the different cells to prevent errors given by 
inconsistent Brillouin zone samplings. Because these discrepancies arise 
only for US pseudos, and they seem to get bigger as the 2 cutoff radii 
in the pseudos grow apart, we have thought that it might be some 
something related to the calculation of the augmentation charges.
I've attached 2 input files as reference for the type of simulation I'm 
running.

Thank you in advance,
Jacopo Masnaghetti.

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