The ABINIT Users Mailing List ( CLOSED )

[Philippe Blaise <philippe.blaise@cea.fr>]

Matteo,

now I feel guilty about doing my benchmarks with a version under 
development, I'm sorry,
and thank to you and also Fabien for your explanations.

Concerning the second part of your email, for my systems : you are right 
20-30 Ha is usually for the wave
functions (for the sigma part indeed); for chi0 and epsilon, 15 Ha or 8 
Ha is a max, only useful for a convergence
study that really shows that a cutoff between 8-10 Ha is generally 
sufficient, and in this case, depending of the
size of the system, cubic or monoclinic, npweps is between 300 - 1000 ; 
the famous findggp was
a problem when I push the cutoffs too high, that is when npweps 
~2000-3000 only to get the last point
of a convergence study.

Regards,

Philippe

Matteo Giantomassi a écrit :
> Dear Philippe and Fabien,
>
> I have introduced the findggp subroutine to find the "irreducible G-pairs",
> namely the set of G-pairs that allow to reconstruct the full dielectric
> matrix, for a given external q-point, using symmetry properties. 
>
> This algorithm will lead to a speed up of the code if the irreducible
> polarizability is calculated using the spectral method where 
> a Hilbert transform must be evaluated for each matrix element.
>  
> Unfortunately it's not possible to improve the performance of the 
> screening part if \chi_0 is obtained through the Adler-Wiser expression 
> using the FFT technique to evaluate the oscillator matrix elements 
> between pseudo-wavefunctions,
> It might be useful, instead, in case of PAW+GW calculations. 
>
> In my development version, I'm using the output of findggp 
> to check the correctness of new pieces of code I'm presently 
> developing, and I'm also working on the optimization of findggp. 
> In my stable release the optimized version of findggp should 
> be called only if the spectral method is employed, thus avoiding 
> the worsening on the performance you are observing.
>
> The problem is that the next stable release of abinit (5.4.4) 
> will not contain these modifications; we agreed with Xavier, indeed, 
> that they will be included starting with version 5.5
> As a consequence, the GW part of abinit-5.4.4 will 
> contain pieces of code that are still under development, sorry for this.
> For the moment the best solution consists in commenting the incriminated 
> loops as you have done.
>
> Continue below
>
>   
>
> In my opinion the scaling should be somewhere between N^3 and N^4, 
> it's not so easy to estimate the scaling of the innner loop on the pairs 
> that have been found since it depends on the degree of symmetry of 
> the system 
> I agree that findgpp could be quite CPU demanding if the number of
> G vectors in \chi_0 is large, but it's possible to optimize the 
> inner loop by ordering the pairs in shell as well as precalculating 
> some arrays.
> The most important problem consists in the fact that findggp 
> cannot be easily parallelized, at least for a fixed q point! 
>
> By the way, how many G vectors are you using to describe the dielectric 
> matrix? I've done tests on systems with a relatively large unit cell 
> (up to 800 G vectors for \chi_0) but they have never 
> required more than few minutes to find the irreducible G-pairs!  
> I guess the cutoff of 20-30 Ha concerns the description of the wavefunctions. 
> Could you report the value of npweps used in your tests?
>
> Regards, 
> Matteo
>
>   
>
>
>
> ----------------
> Matteo Giantomassi
> PCPM/FSA/UCL
>
> -------------------------------------------------
> This mail sent through IMP: atlas.pcpm.ucl.ac.be
>
>