# CUTOFF convergence problem

Matt W MattWa... at gmail.com
Tue Nov 10 09:13:09 UTC 2015

```Hi Katrina,

At a given geometry (the one in your input) the total energy of the system
converges quite OK with cutoff after about 800 Ry, according to me.

Are you comparing the energy differences after performing geometry
optimization? Is it possible that a some of your runs are optimizing to
subtly different structures. Even a tiny difference in the shape of the
potential energy surface can lead you into a different minima, are you are
starting from quite high energy structures it seems. Your structure 2 seems
well behaved, but structure 1 gives a weird energy at the highest cutoff
you used.

Matt

BTW, the basis sets you are using DZV-GTH are probably not really
production quality.

On Monday, November 9, 2015 at 3:33:32 PM UTC, katarína stančiaková wrote:
>
> Dear CP2K users,
> I am running my calculations and I have CUTOFF convergence problem - in
> fact I can not see any convergence in the total energy. I know that the
> relative differences in energies are important (not the absolute values of
> energies), therefore I am comparing the energy differences between two
> structures (I am working with zeolites, and my two structures differs in
> the position of hydrogen). Even with very high cutoff values I can not
> observe any convergence of the total energies as well as energy differences.
> Do you have any idea where is the problem? I will be thankful for any help!
>
> Here are my data:
>
> CUTOFF [Ry]            Total Energy [a.u.]
>                                 structure 1                structure 2
> 600                           -3467.0403965424    -3467.0619253888
> 800                           -3467.0521866655    -3467.0622253178
> 1000                         -3467.0838714739    -3467.1096147317
> 1200                         -3467.0833499728    -3467.1103018678
> 1400                         -3467.0975445683    -3467.1102723936
>
>
> ENERGY DIFFERENCES
>
>
> CUTOFF [Ry]    E_str1 - E_str2 [kJ/mol]
>
> 600                55.9750007029
> 800                26.1004960059
> 1000              66.932470253
> 1200              70.0749269477
> 1400              33.0923456248
>
> Thank you
> Katarina
>
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