[QE-developers] Parallelization question for G-wise projector-wavefunctions product computation

[Stefano de Gironcoli]

Dear Luca,

    the array betapsi_g(1:npw,1:nkb,1:nbnd) looks very big to me, unless 
the system is very small

    what do you need it for ? maybe one can compute the target quantity 
on the fly rather than storing this intermediate result


stefano


On 08/02/25 15:31, Marin Luca wrote:
> Dear QE developers,
> I had a conceptual question of how I should correctly modify a 
> subroutine in order to avoid parallelization and memory’s absurdities. 
> Specifically, my idea is to slightly modify one of the subroutines in 
> the “calbec” (becmod.f90) interface that is computing <\Beta|\psi> 
> products, and I am using QE v.6.4 because I’m working on a specific 
> code repository using this version. By calling such or similar line:
> CALL ZGEMV( 'C', npw, nkb, (1.0_DP,0.0_DP), beta, npwx, psi, 1, &
> (0.0_DP, 0.0_DP), betapsi, 1 )
> and subsequently summing through bands:
> CALL mp_sum( betapsi( :, 1:m ), intra_bgrp_comm )
> “calbec_k” computes the betapsi product as a 2D matrix with dimension 
> (number of projectors, number of bands). My goal is to compute the 
> same product but accessing the elements at each G vectors, i.e. 
> without summing over the G vectors themselves. To this end, my (very 
> basic, probably terrible) idea was to define another subroutine with 
> an extra variable like this (without breaking the requirement of 
> having unambiguous interface procedures in “calbec”):
> calbec_k_modified ( npw, beta, psi, betapsi, betapsi_g, nbnd )
> COMPLEX (DP), INTENT (out) :: betapsi_g(:,:,:)
> and explicitly computing betapsi_g with nested do loops:
> DO k = 1, npw
> DO i = 1, nkb
> DO j = 1, m
> betapsi_g(k, i, j) = CONJG(beta(k, i)) * psi(k, j)
> ENDDO
> ENDDO
> ENDDO
> I am very unsure of whether this is a feasible approach and how I 
> should handle the MPI parallelization in this case (I have little 
> experience in QE development). I read the parallelization section of 
> the developer’s manual, but I’m still quite confused. Any suggestions, 
> critics or explanations will be very appreciated.
> Best regards,
> Luca Marin
> ETH Zurich, PhD student
>
>
>
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