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

[Lorenzo Paulatto]

I think that code was written before bands parallelizations was viable. 
Personally, I do not understand the current implementation of bands 
parallelization and I won't have the quiet required to immerse in this 
kind of complications until schools resume. Ff you manage to fix the 
code it is very much appreciated :)

Thank you for reporting in any case!


On 3/28/20 7:50 PM, Morales-Silva, Miguel A. wrote:
> Regarding #1, the contributions from ionode to the energy and to deexx 
> are properly broadcasted,
> the problem comes with pool and band parallelization in exx. In this 
> case, ionode only operates
> on a subset of the (xkq,xkp) kpoint pairs and on a subset of the 
> (ibnd,jbnd) band pairs.
> Contributions not assigned to ionode are not included. This can be 
> easily seen by accumulating
> the result from PAW_xx_energy on a separate variable and looking at its 
> value as a function of npools.
> Results clearly vary with the number of kpoint pools. These 
> contributions are typically so small, that at
> the end it probably doesn't matter.
> 
> Regarding #2, it is correct that the problem is in the charge 
> augmentation, common to both PAW and USPP.
> There are clear differences in runs with nkpts>1 with and without tqr. 
> The augmentation functions in real space
> (e.g. |tabxx(ia)%qr|(...)) do not depend on the specific k-points of the 
> orbital pair densities being augmented.
> This, I believe, is incorrect. The phase factor coming from (xkp-xkq) is 
> added in addusxx_g, but not in addusxx_r.
> 
> best,
> 
> *************************************
> Miguel A. Morales
> Quantum Simulations Group, Physics Division
> Lawrence Livermore National Laboratory
> phone: 925-423-4956
> *************************************
> 
> ------------------------------------------------------------------------
> *From:* developers <developers-bounces at lists.quantum-espresso.org> on 
> behalf of Lorenzo Paulatto <paulatz at gmail.com>
> *Sent:* Saturday, March 28, 2020 10:00 AM
> *To:* developers at lists.quantum-espresso.org 
> <developers at lists.quantum-espresso.org>
> *Subject:* Re: [QE-developers] questions and possible bug on exx 
> calculations
> I add to what already said by Paolo
> 
>>  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.
> 
> The code is only executed in ionode, and the result added into
> deexx,later on in exx.f90 there is
> 
> IF (okvan) THEN
>       CALL mp_sum( deexx(:,ii), intra_egrp_comm )
> ENDIF
> 
> Which means that if we compute that part on all CPUs, the contribution
> would be multiplied by the number of CPUs. It would be better to majke
> PAW_newdxx parallel, but as it is inexpensive I did not bother. Bugs are
> always possible. Same story of PAW_xx_energy, which is added to energy,
> which is then summed over all CPUs
>       CALL mp_sum( energy, inter_egrp_comm )
>       CALL mp_sum( energy, intra_egrp_comm )
>       CALL mp_sum( energy, inter_pool_comm )
> 
> 
> 
>>  2.  From what I see, tqr=.true. is not implemented for exx routines
>>     when k-points are used.
> 
> It is implemented. To clarify: there is a part of code that is common to
> PAW and Ultrasoft, this part depends on the value tqr and is computed by
> either newdxx_r or newdxx_g. Than there is a part which is paw-only,
> which does not depends on the value of tqr; this is computed by
> PAW_newdxx. Also note that when okpaw=.true. also okvan is true.
> 
> 
>>     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.
> 
> There are probably some bugs in that part, but not what you think.
> Actually as the PAW specific part is identical with and without tqr, I
> would suspect the bug is in the Ultrasoft code.
> 
> 
> cheers#
> -- 
> Lorenzo Paulatto - Paris
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-- 
Lorenzo Paulatto - Paris