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

Patrick Gono patri... at gmail.com
Wed Dec 4 10:27:04 UTC 2019

```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 <sing... 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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