[gpaw-users] odd SCF convergence behavior

[Nichols A. Romero]

Jussi,

I had not seen this ticket. Is it still a problem in the current
version of GPAW?

----- "Jussi Enkovaara" <jussi.enkovaara at csc.fi> wrote:

> Nichols A. Romero wrote:
> > Hi,
> > 
> > Here is the input file in question.
> > http://en.pastebin.ca/1973390
> > 
> > The SCF cycle is well-behaved at 64 MPI tasks,
> > http://en.pastebin.ca/1973391
> > 
> > but is poorly behaved at 32 MPI tasks. 
> > http://en.pastebin.ca/1973392
> > 
> > Is there a simple way to explain this difference?
> 
> Hi,
> I assume that this might be related to the ticket 51 
> https://trac.fysik.dtu.dk/projects/gpaw/ticket/51
> 
> The problem is/was that with systems with symmetries i.e degenerate
> electronic
> states, very tiny differences (e.g. 10^-12) in LCAO Hamiltonian
> matrix, arising 
> from different number of CPUs, result in completely different
> eigenvectors for the 
> degenerate states. Basically, they are just different linear
> combinations, so 
> physically there is no problem, however, the different values affect
> the numerics 
> and differences e.g. in the total energy start to accumulate a little
> by little. 
> For example, in your output one can see that the total energies for
> the 32 task and 
> 64 task cases remain the same for couple of first iterations.
> 
> You could try to check whether the initialization is the issue by
> writing the wave 
> functions to a restart file after the first iteration, and then
> restarting from the 
> same file both with 64 task and 32 task cases.
> 
> After I found the problem I was not that worried as the converged
> total energies 
> were in good agreement with different CPU counts, and also the number
> of iterations 
> differed only by 1-4 (after all, it was just slightly different
> starting guess for 
> the wave functions).
> 
> If also your problem is related to initialization it is of course
> quite worrying if 
> the convergence behaviour is affected so much.
> 
> Best regards,
> Jussi

-- 
Nichols A. Romero, Ph.D.
Argonne Leadership Computing Facility
Argonne National Laboratory
Building 240 Room 2-127
9700 South Cass Avenue
Argonne, IL 60490
(630) 252-3441