[gpaw-users] odd SCF convergence behavior

[Jussi Enkovaara]

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