CUTOFF convergence problem

[Matt W]

Dear Katrina,


You are right, I am comparing the energy differences after geometry 
> optimizations and it seems that with different cutoff values I am finding 
> different minima. What is bothering me is that even CUTOFF 1200 Ry is 
> obviously not enough as I found another minima for structure 1 with cutoff 
> 1400 Ry. The using of such high cutoff is not very convinient for zeolite 
> structures as far as I know. So I am looking for some "bug" in my setup.
>
>
energy minimisation can be essentially chaotic, tiny differences in the 
potential energy surface / initial conditions etc can rapidly lead you to 
minimise to different minima, especially if you start from very high energy 
structures. I think the 'bug' in your set up is that you start from a very 
high energy state and then minimize. If you did this for many different 
starting positions you should locate all the possible minima eventually. 
But better to start from close to a local minima and not ask so much from 
the optimiser.

It is not really a question of one being right and the other wrong in this 
case if you find different minima. Do both minima exist for both 1200 and 
1400 Ry cutoff with consistent energy differences?

HTH,

Matt
 

> Katarina
>
> Dňa utorok, 10. novembra 2015 10:13:10 UTC+1 Matt W napísal(-a):
>>
>> 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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