[Pw_forum] Difference in total energies (serial vs parallel) for calculations with hybrid functional using norm conserving pseudopotential: Possible bug

[Prasenjit Ghosh]

Dear all,

We are doing calculations for my system (PbI2) with hybrid functionals
(using the ACE algorithm) and norm conserving pseudopotentials. Presently
we are performing this calculations using scalar relativistic
non-conserving pseudopotentials for Pb and I. The pseudopotentials have
been generated by me using PBE functionals. I have tested these (at the
level of GGA) and it can reproduce well the lattice parameters and
electronic bandstructure of a range of different compounds (from metallic
Pb to ionic PbI2 and CsPbBr3).

When we perform the hybrid calculations using GAU-PBE, we find that the
total energy differs when we use 1 core and 4 cores. The energy difference
is about 0.23 Ry. However, for both the cases, the standard GGA-PBE
calculations give same results (total energy). Even the band gap with
GAU-PBE (using 1 and 4 cores) are same. A look into the output file
suggests that the difference comes from
-averaged Fock potential + Fock energy.

Since I have generated the pseudopotential, I thought that there might be
some issue with this. Hence to cross check, I downloaded the normconserving
pseudopotentials for Pb and I from http://theossrv1.epfl.ch/Main/
Pseudopotentials (pbe.0.3.1.tgz). For these two pseudopotentials also the
total energies (1 core vs 4 cores) differ. The input files, pseudopotential
files and the output files can be downloaded from the link given below:

http://www.iiserpune.ac.in/~pghosh/QE/

These results are reproducible on different machines, using different
compilers (eg. gfortran vs intel).

I think that there is a bug in the code, particularly the part where the
contributions to total energy from EXX is collected. If the developers can
suggest where exactly to look for in the code, I can give it a try to sort
it out.

With regards,

Prasenjit.

-- 
PRASENJIT GHOSH,
IISER Pune,
Dr. Homi Bhabha Road, Pashan
Pune, Maharashtra 411008, India

Phone: +91 (20) 2590 8203
Fax: +91 (20) 2589 9790
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