[CP2K-user] How does work on the EXTERNAL_POTENTIAL

[ning zhang]

Hi Everyone,

I am learning the metadynamics by the website: 
https://www.cp2k.org/exercises:2015_cecam_tutorial:mtd1

However, there is a question on the CP2K_INPUT 
<https://manual.cp2k.org/trunk/CP2K_INPUT.html>/FORCE_EVAL 
<https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL.html>/
EXTERNAL_POTENTIAL 
<https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/EXTERNAL_POTENTIAL.html>. 
In the first task of tutorial, an example on the dynamics of two HNO3 
molecules over a graphene sheet are considered. If a constraint is not 
added, it is observed that the two molecules very fast and expolre a large 
spece of configurations, being often far from each other and far from 
graphene. In order to restrict the exploration to region, it is necessary 
to limit the movement of the two molecules. To this purpose, an external 
potential defining a spherical potential centered on the center of the 
system coordinated is added. The potential is given as follows:

 &EXTERNAL_POTENTIAL
     ATOMS_LIST   61..70
     FUNCTION   0.000000000001*(Z^2)^4
  &END

  &EXTERNAL_POTENTIAL
     ATOMS_LIST   61..70
     FUNCTION   0.0000000000001*(X^2)^4
  &END

  &EXTERNAL_POTENTIAL
     ATOMS_LIST   61..70
     FUNCTION   0.0000000000001*(Y^2)^4
  &END


Question: How does work on such external_potential? i.e., How does such 
external_potential constaint the two HNO3 molecules to the vicinity 
of graphene?

Ning
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