[CP2K-user] Energy oscillations in vdW-DF calculations

[Michael Fischer]

Dear CP2K community,

I am seeing a somewhat strange behaviour in CP2K calculations using vdW-DF 
functionals (version 7.1): Within one SCF cycle, the calculation sometimes 
reaches a point where it jumps back and forth between different energy 
values. Such behaviour is illustrated in a portion of a log file appended 
to this message. This calculation used the vdW-DF-b86r functional, but I 
have seen the same behaviour with other vdW-DF functionals. 
I should add the following points:
1) The compound under consideration, a zeolite with an adsorbed molecule, 
contains only main group elements (Si, O, C, H, N).
2) Analogous calculations using DFT-D methods (like PBE-D3) show no 
convergence issues whatsoever.
3) A change of preconditioner (FULL_ALL/FULL_KINETIC) seems to affect the 
behaviour, but does not resolve the issue.

I should also note that I tested the "CUTOFF" parameter within &XC 
&vdW_POTENTIAL &NON_LOCAL part of the input and found that it should be 
increased with regard to the default setting to get converged adsorption 
energies (I use 400 Ry).

Any help or suggestions would be highly appreciated.
Best regards,
Michael

Portion of output - oscillations start in the 8th iteration
  Step     Update method      Time    Convergence         Total energy    
Change
  
------------------------------------------------------------------------------
     1 OT DIIS     0.15E+00    5.0     0.00004571     -5301.9144127012 
-5.30E+03
     2 OT DIIS     0.15E+00    2.4     0.00001858     -5301.9158989932 
-1.49E-03
     3 OT DIIS     0.15E+00    2.4     0.00001214     -5301.9161596822 
-2.61E-04
     4 OT DIIS     0.15E+00    2.5     0.00000142     -5301.9161913868 
-3.17E-05
     5 OT DIIS     0.15E+00    2.4     0.00000043     -5301.9161930782 
-1.69E-06
     6 OT DIIS     0.15E+00    2.4     0.00000011     -5301.9161943445 
-1.27E-06
     7 OT DIIS     0.15E+00    2.4     0.00000005     -5301.9161967775 
-2.43E-06
     8 OT DIIS     0.15E+00    2.4     0.00000001     -5301.9161955633  
1.21E-06
     9 OT DIIS     0.15E+00    2.4     4.0079E-09     -5301.9161967782 
-1.21E-06
    10 OT DIIS     0.15E+00    2.4     1.7554E-09     -5301.9161955634  
1.21E-06
    11 OT DIIS     0.15E+00    2.4     6.2727E-10     -5301.9161967782 
-1.21E-06

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