cut-off convergence, what am I doing wrong?

[David T]

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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