[CP2K:8181] cut-off convergence, what am I doing wrong?

[Iain Bethune]

Dear David,

Unlike in SCF (where the energy is variational), the energy will converge with respect to the cutoff, but not necessarily downwards e.g. oscillation is possible.  You can see this behaviour in the example on the website:  https://www.cp2k.org/howto:converging_cutoff

So nothing to worry about, just choose CUTOFF/REL_CUTOFF settings that give you an accuracy you are happy with.

- Iain

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Iain Bethune
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> On 22 Sep 2016, at 13:54, David T <amazing... at gmail.com> wrote:
> 
> Dear all
> 
> I am testing the cut-off convergence of a MOF, 500 atoms containing Zn, C, O and H.
> I used  BASIS_SET DZVP-MOLOPT-SR-GTH and POTENTIAL GTH-PBE
> I run some script testing both CUTOFF and REL_CUTOFF (script attached)
> 
> Analysing the results I found something which I don't understand and I guess it's is wrong.
> 
> For instance REL_CUTOFF fixed to 40 and scanning CUTOFF from 300 to 600, the energy instead of going down, it increases (namely become less negative). 
> 40-300.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.665718760408708
> 40-350.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.634747854958732
> 40-400.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.603373347244997
> 40-450.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.594127171910259
> 40-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592618500900244
> 40-550.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.585887919147353
> 40-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.587025639214517
> 
> fixing CUTOFF to and scanning REL_CUTOFF from 40 to 60, the energy oscillates
> 
> 40-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592618500900244
> 45-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592620523051664
> 50-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592620119527965
> 55-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592616871606879
> 60-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592613127000732
> 
> This is always the case meaning there is some systematic error in my protocol :-S 
> 
> 60-300.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.665685380023206
> 60-350.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.634703110548799
> 60-400.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.603299460428389
> 60-450.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.593993930518991
> 60-500.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.592613127000732
> 60-550.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.585353744683744
> 60-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.586701438765886
> 
> 40-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.587025639214517
> 45-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.586747652509075
> 50-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.586702299202443
> 55-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.586701458000789
> 60-600.out: ENERGY| Total FORCE_EVAL ( QS ) energy (a.u.):            -4278.58670143876588
> 
> any suggestion?
> 
> Cheers
> Davide
> 
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