Defining a CV for protonation from water to adsorbate

[riyazpu]

Hi
I’m trying to use PLUMED OPES_META to study the dynamics of CO protonation on various metal surfaces in a solvent environment. Specifically, I am trying to compute the energy landscape for CO protonation from nearby water molecules.

The challenge I’m facing is that, during sampling, when I allow all water molecules to act as proton sources by biasing the adsorbate coordination with H of water, the added proton often drifts away from COH after protonation and sits on the surface, making it difficult to sample the CO–H–OH configuration multiple times. To address this, I’ve had to add several constraints in the PLUMED script to focus only on CO–H bond formation from a single H₂O molecule. However, when it *COH the OH abstracts a proton from the next neighbour (Grotthus mechanism), forming H2O. When the proton of *COH dissociate, it forces the H of near H2O to dissociate, which then sometimes binds to the surface. This significantly distorts the energy profile.

Would anyone happen to know of a cleaner or more robust approach for sampling only the protonation step from water to adsorbate, such that H does not adsorb to the surface during sampling? Any suggestions on refining the collective variables or simulation setup would be greatly appreciated.

Best regards,
Riyaz