[CP2K-user] Selecting cutoff from the Energy and NGRID data

[Ramanish Singh]

Hi,

I am simulating a system of 64 Iodine atoms. I performed the cutoff test to 
find the suitable cutoff and reliable_cutoff but the data I got is somewhat 
weird. I have attached the data with this post.
For example, for REL_CUTOFF=30 I got the following data: 

# Number of Grids: 4

# Relative Cutoff (Ry): 30

# Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on 
grid 3 | NG on grid 4

    200.00  -725.7833908007       0   15624   15391    4590

    250.00  -725.7860435954       0   12298   13049   10258

    300.00  -725.7849325650       0    7090   18177   10338

    350.00  -725.7837028953       0    3652   19891   12062

    400.00  -725.7839064559       0    2188   17023   16394

    450.00  -725.7853932286       0    1619   17592   16394

    500.00  -725.8084189293       0     527   16090   18988

    550.00  -725.7834120461       0       0   16617   18988

    600.00  -725.7833214512       0       0   15624   19981

    650.00  -725.7826948381       0       0   15624   19981

    700.00  -725.7826948381       0       0   15544   20061

    750.00  -725.7844897119       0       0   12298   23307

    800.00  -725.7829016340       0       0   12218   23387

The energy is exactly the sane for CUTOFF = 650 and 700 Ry , but it changes 
for CUTOFF=750 and 800 ( with error more than 1e-3).
Similar thing happens for other values of REL_CUTOFF as well. The error 
decreases to 1e-7 as CUTOFF increases but suddenly jumps to 1e-3 when 
CUTOFF is increased beyond 600 Ry.
Do I need to increase my CUTOFF beyond 800 to see if it converges? And is 
it worth it to keep it that high keeping in mind the increase in the 
simulation time?

Thanks,
Ramanish
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