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

Travis polla... at gmail.com
Wed Dec 4 16:19:46 UTC 2019

```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
> <javascript:>> 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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