[gpaw-users] Wavefunction convergence problem

[Gaël Donval]

Hi,

I have generated a quite a big slab (80 atoms, time reversal symmetry
only) with ASE and try to perform structure optimization on it.

        Unit Cell:
                   Periodic     X           Y           Z      Points  Spacing
          --------------------------------------------------------------------
          1. axis:    no    23.278154    0.000000    0.000000   128     0.1819
          2. axis:    yes    0.000000    7.679180    0.000000    40     0.1920
          3. axis:    yes    0.000000    0.000000    8.145000    48     0.1697


I use only one k-point (gamma).

When I first tried to get SCF convergence, I couldn't even converge to
anything, even with very low mixing parameters. I had to remove bands
(from nelectron to 25 unoccupied bands) to get the convergence that you
will see bellow.

At the first relaxation step, in the SCF calculation, everything
converges smoothly down to some point then hits a limit: the error on
the density stays around 10^-4 and the error on the wavefunction is
switching between two values. For instance:

        iter:  47  10:02:07  -3.19  -4.20    -356.939274  2      2      
        iter:  48  10:02:09  -3.11  -4.13    -356.938830  2      3      
        iter:  49  10:02:12  -3.19  -4.43    -356.938031  2      2      
        iter:  50  10:02:14  -3.11  -4.49    -356.937563  2      2      
        iter:  51  10:02:17  -3.19  -4.60    -356.937381  1      1      
        iter:  52  10:02:20  -3.11  -4.57    -356.937543  1      1      
        iter:  53  10:02:22  -3.19  -4.59    -356.938149  1      1      
        iter:  54  10:02:25  -3.11  -4.56    -356.937988  1      1      
        iter:  55  10:02:27  -3.19  -4.53    -356.938700  2      2
        ...
        iter: 283  10:12:19  -3.15  -3.84    -356.893178  2      2      
        iter: 284  10:12:22  -3.08  -4.34    -356.882750  1      1      
        iter: 285  10:12:24  -3.15  -4.55    -356.858456  2      2      
        iter: 286  10:12:27  -3.08  -4.24    -356.864220  2      1
        

If I consider that this is enough and stop there, then I got the same
behavior in the second step with the error on the density converging to
~10^-4 *but* the error on the wavefunctions stays at ~10^-1.4 (same
behavior, switches between two values).

I followed the recommandations on the "Convergence issues" web page,
without success. I also tried different mixers and eigensolvers.

Does anyone has an idea about that?

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