[CP2K-user] [CP2K:22125] CP2K constrained MD: O–H coordination number constraint does not prevent OH bond breaking

[Qi Zhang]

Hello CP2K developers and users,

I am running a constrained MD simulation in CP2K and trying to keep the O–H 
coordination number (CN) of hydroxyl groups constrained during the 
trajectory. However, the constraint does not seem to hold as expected.

My setup is:

&COLVAR
&COORDINATION
KINDS_FROM Oh
KINDS_TO H
R_0 [angstrom] 1.25
NN 8
ND 16
&END COORDINATION
&END COLVAR

&CONSTRAINT
&COLLECTIVE
COLVAR 1
INTERMOLECULAR
TARGET 0.88
&END COLLECTIVE
&LAGRANGE_MULTIPLIERS
COMMON_ITERATION_LEVELS 1
&END
&END CONSTRAINT

My intention is to constrain the O–H CN so that each hydroxyl oxygen keeps 
its single bonded H during the MD run.

However, after about 15 ps of simulation, I found from the trajectory 
analysis that the CN is not actually maintained. Around ~8 ps, some O atoms 
in OH groups lose their only H, so their O–H CN effectively becomes 0.

To diagnose this, I checked:

   - 
   
   the O–H distances in the OH groups
   - 
   
   the number of H atoms around each hydroxyl O as a function of time
   
Both analyses suggest that some hydroxyl O atoms indeed lose their bonded H 
during the simulation, even though the collective variable is supposed to 
be constrained.

My questions are:

   1. 
   
   Is this the correct way to constrain an intramolecular O–H coordination 
   number in CP2K?
   2. 
   
   Could the use of INTERMOLECULAR here be incorrect for this purpose?
   3. 
   
   What should I do to correctly constrain the O–H coordination number to 
   1? 
   
I will attach the figures and the input file showing: 

   - 
   
   O–H bond distances vs time
   - 
   
   the number of H atoms around each hydroxyl O vs time
   
Any suggestions would be greatly appreciated. Thank you very much for your 
help.

Best regards

[image: OH_distances_vs_time.png]

[image: O_H_count_vs_time.png]

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