[CP2K-user] [CP2K:12592] Choosing cutoff for AIMD

[Ramanish Singh]

Thank you so much Patrick and Travis! 

On Wednesday, December 4, 2019 at 10:19:46 AM UTC-6, Travis wrote:
>
> Hi,
>
> Just to add, the 40 referenced in "Now assuming this is the highest among 
> all elements present, take this number and multiply it by 40" should be set 
> to whatever REL_CUTOFF is. I typically use a value of 50 for static cell 
> calculations and 80 for variable cell ones. These figures are based on my 
> experience and from literature on condensed phase simulations. Using the 
> basis set listed in Patrick's post, this would give cutoffs of 520 Ry and 
> 850 Ry with some rounding up to 'nicer' numbers. The point of this is to 
> ensure that the tightest orbitals make it to the finest grid. Importantly, 
> this assumes you don't use the smoothing function in 
> https://manual.cp2k.org/trunk/CP2K_INPUT/ATOM/METHOD/XC/XC_GRID.html.
>
> -T
>
>
>
> On Wednesday, December 4, 2019 at 6:27:18 AM UTC-4, Patrick Gono wrote:
>>
>> Dear Ramanasish,
>>
>> You are attempting to do ab-initio molecular dynamics. At each step, the 
>> forces are calculated by performing a single point energy calculation of 
>> your system. If your CUTOFF is too small, the single point calculation 
>> leads to the wrong electron density, and hence your forces will also be 
>> wrong.
>>
>> Choosing CUTOFF, you only need to make sure that your energy in a single 
>> point calculation is converged. To be completely clear: the latter of the 
>> two options you propose.
>>
>> In general, look at the highest exponent of the basis you use. For 
>> example, if your system contains oxygen, and you use the double-zeta MOLOPT 
>> basis set:
>>  O  DZVP-MOLOPT-SR-GTH DZVP-MOLOPT-SR-GTH-q6
>>  1
>>  2 0 2 5 2 2 1
>>      10.389228018317  0.126240722900  0.069215797900 -0.061302037200 
>> -0.026862701100  0.029845227500
>>       3.849621072005  0.139933704300  0.115634538900 -0.190087511700 
>> -0.006283021000  0.060939733900
>>       1.388401188741 -0.434348231700 -0.322839719400 -0.377726982800 
>> -0.224839187800  0.732321580100
>>       0.496955043655 -0.852791790900 -0.095944016600 -0.454266086000 
>>  0.380324658600  0.893564918400
>>       0.162491615040 -0.242351537800  1.102830348700 -0.257388983000 
>>  1.054102919900  0.152954188700
>>
>> The highest exponent is 10.389... in the first row of values, on the very 
>> left.
>>
>> Now assuming this is the highest among all elements present, take this 
>> number and multiply it by 40. That should give you some idea of the 
>> appropriate value of CUTOFF, in this case CUTOFF = 400. Run a few ENERGY 
>> calculations with values, say, between 350 and 600, and check that the 
>> energy is, indeed, converged.
>>
>> Yours sincerely,
>> Patrick Gono
>>
>> On Tue, Dec 3, 2019 at 10:42 PM Ramanish Singh <si... at umn.edu> wrote:
>>
>>> Hi,
>>>
>>> I have a question regarding the plane wave CUTOFF in the MGRID section. 
>>> I am doing NVT MD simulations with a liquid box.
>>>
>>> For deciding the CUTOFF value do I need to run the MD simulations at 
>>> different values of CUTOFF and then decide the appropriate CUTOFF based on 
>>> evolution of energy with time for different CUTOFF values
>>>
>>> OR
>>>
>>> a single point energy of the system (RUN_TYPE ENERGY) is fine? (as 
>>> given here: https://www.cp2k.org/howto:converging_cutoff )
>>>
>>> Please also give any relevant references for the same if available.
>>>
>>>
>>> Thanks,
>>> Ramanish Singh
>>>
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>>
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