[CP2K-user] [CP2K:11921] Selecting cutoff from the Energy and NGRID data

[Ramanish Singh]

Dear Patrick,

Thank you very much for your suggestion. 
I still have a doubt. I think the peculiar behavior is also seen for 
REL_CUTOFF=60. The convergence is 1e-7 for CUTOFF=550 but after that it 
suddenly decreases to 1e-4 and 1e-3 at CUTOFF=750. So shall I go with 500 
or with 800 for CUTOFF. 

Thanks,
Ramanish Singh

On Saturday, June 29, 2019 at 8:53:20 AM UTC-5, Patrick Gono wrote:
>
> Dear Ramanish,
>
> Looking at the rest of your data, it appears that the problem was a value 
> of the relative cutoff that was too small. For example, if you consider the 
> convergence of energy with respect to the total cutoff for REL_CUTOFF = 60, 
> the suspicious behaviour vanishes and even small values of CUTOFF yield 
> converged energies.
>
> I would suggest you stick with REL_CUTOFF = 50 or 60. These are also the 
> values suggested in the input manual to get accurate results. For the 
> absolute cutoff, you are better off using higher values than the default 
> one as well. Some even suggest using values as high as 800 or 1000 Ry, if 
> accuracy is of the highest importance. The correct choice of the value will 
> also depend on the system (e.g. its size, or the elements present). In any 
> case, from your tests it appears that smaller values of CUTOFF are 
> sufficient as long as you use REL_CUTOFF = 50 or higher, though.
>
> What I personally do is start with a reasonably high value for CUTOFF 
> (e.g. 700 or 800 Ry). If I find that a few extra percent of performance are 
> critical for my calculations, I might test a smaller value for the CUTOFF 
> by looking at the quantity of my interest. For example, if I were to study 
> binding energies of small molecules on surfaces, I would study the effect 
> of a smaller value of CUTOFF on that exact quantity. If I find that the 
> effect is negligible, I might use a smaller cutoff (e.g. 500, or even 350 
> Ry).
>
> However, from my experience, other parameters, such as the exchange and 
> correlation functional used, or the basis sets, have a much stronger effect 
> on the computational resources required.
>
> Hope this helps.
> Yours sincerely,
> Patrick Gono
>
>
>
> On Thu, 27 Jun 2019 at 16:03, Ramanish Singh <si... at umn.edu 
> <javascript:>> wrote:
>
>> 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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