[CP2K-user] Apply external force on a slab system

[Nam Tran]

Dear CP2K users and experts.

I am trying to simulate the interaction of a surface with a liquid layer 
under a high pressure using ab initio molecular dynamics. The system is 
illustrated in the figure below (I have some specific reasons that I need 
to use vacuum). In order to apply the load to the top slab an external 
force acting on the layer is needed. 

[image: Slide1.jpeg]

I found in the tutorial by Prof. Marcella (
https://www.cp2k.org/exercises:2015_cecam_tutorial:mtd1) using external 
potential (*CP2K_INPUT / FORCE_EVAL / EXTERNAL_POTENTIAL 
<https://manual.cp2k.org/trunk/CP2K_INPUT/FORCE_EVAL/EXTERNAL_POTENTIAL.html>*) 
to constrain the motion of molecules. From my understanding the external 
potential will act on each atom with a force of F=-dE/dr. So my idea is 
using the command to setup a constant external force to the top atomic 
layers.

&EXTERNAL_POTENTIAL
      ATOMS_LIST   1..20
      FUNCTION (A/B)*Z
      PARAMETERS A B
      UNITS [eV] [angstrom]
      VALUES 0.5 1.0
&END EXTERNAL_POTENTIAL

That would applied a constant normal (along z direction) force of 0.5 eV/A 
(this value is chosen randomly) to the chosen atoms. I wonder if the above 
setting will work?

Please correct me if I have some misunderstanding. 
I appreciate any advises and recommendations.

Regards
Nam
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