[CP2K-user] [CP2K:11930] Selecting cutoff from the Energy and NGRID data

[Ramanish Singh]

Thank you so much Patrick. 



On Sunday, June 30, 2019 at 6:34:37 AM UTC-5, Patrick Gono wrote:
>
> Dear Ramanish,
>
> In that particular case, the difference between 700 Ry and 750 Ry is on 
> the order of 1e-6: (725.7828004716 - 725.7827990986 = 0.000001373). That's 
> 1e-5 in eV. I think it's safe to say we are converged.
>
> The convergence might be easier to establish if you were to plot the 
> calculated values.
>
> Yours sincerely,
> Patrick Gono
>
> On Sat, 29 Jun 2019 at 17:01, Ramanish Singh <si... at umn.edu 
> <javascript:>> wrote:
>
>> 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> 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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