[QE-developers] questions and possible bug on exx calculations

[Morales-Silva, Miguel A.]

Dear developers,

I apologize if this is incorrect or discussed somewhere else, but I couldn't find anything.
I have a a few comments/questions about the exx routines in QE.


  1.  On routines PAW_newdxx and PAW_xx_energy, the code is only executed on ionode which I believe is a bug when k-point and/or band parallelization is used.
  2.  From what I see, tqr=.true. is not implemented for exx routines when k-points are used. The phases of the pair densities need be added, probably inside addusxx_r for this to be correct. In a handful of cases I've tested, I get significantly different results with/without tqr with hybrid functionals and kpoints. Gamma point or single kpoint runs, of course, is fine. It would probably be better to perform a runtime check and abort if necessary.
  3.  Many of the executables in the PP/src folder rely on read_file to initialize/setup the calculation. The routine init_us_1() is called inside. When uspp/paw are used in combination with finite kpoints in qvan_init(...) (e.g. as in exx routines), qnorm needs to be set correctly to get the interpolation grid of qrad set properly. Unfortunately, I can't calculate the necessary value of qnorm without knowing the kpoints, which are also read in read_file. Right now, I am forced to use an upper bound to qnorm (using 3.5), which I set before calling read_file. This might be fine, since the overhead is negligible and it probably doesn't matter in the bigger scheme of things, but it would be nice to be able to control this (the value of qnorm or the range of interpolation) more transparently/easily. Just a wish!

best regards and apologies if I'm wrong in any of my comments above,


*************************************
Miguel A. Morales
Quantum Simulations Group, Physics Division
Lawrence Livermore National Laboratory
phone: 925-423-4956
*************************************
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