[CP2K-user] [CP2K:21531] Adding capping hydrogens for unstable oxygens in QM/MM simulation - Virtual Site approach?

[Ben siu]

Dear CP2K Community,

I'm running QM/MM simulations where several oxygen atoms (some inside and 
some outside of my QM region) are exhibiting instability. I'd like to 
stabilize these oxygens by adding capping hydrogens "on the fly" without 
modifying my original structure files. 

Based on my reading of the documentation, the VIRTUAL_SITE functionality 
might be applicable, but I'm uncertain about the correct implementation. My 
specific questions are:

   1. 
   
   Is it possible to use VIRTUAL_SITE to add capping hydrogens to stabilize 
   dangling oxygens in the QM region?
   2. 
   
   If so, what is the correct syntax? I've attempted to understand the 
   documentation but remain unclear on:
   - How to properly define the position of the virtual hydrogen relative 
      to the oxygen
      - How to include this virtual site in the QM region
      - How to set appropriate restraints to maintain proper O-H geometry
      - Whether to not I have to modify the Coordinate and forcefields
   3. 
   
   Alternatively, is there a better approach in CP2K for addressing 
   unstable oxygens during a QM/MM simulation?
   
For context, I'm using CP2K version 7.1 with a QMMM setup where the QM 
region is treated with DFTB3 and the MM region with AMBER force fields.

Any guidance or examples would be greatly appreciated.

Thank you,
Ben

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