I use the material studio software to research water splitting by a single transition metal electrocatalyst.
Dear Ghazaleh,
I think a negative slope between *OH and *OOH sounds very strange and I would advise you to be very careful with your results. In my experience this scaling relation is very robust and does not change when considering different systems. The same scaling relations have been observed independent of whether transition metal doped graphene (DOI: 10.1016/j.nanoen.2016.04.011), transition metal oxides (DOI: 10.1002/cctc.201000397) or homogeneous catalysts (DOI: 10.1021/acs.jpcc.8b03935) were considered. E.g. in all cases it was roughly
Delta G(*OOH) = Delta G(*OH) + 3.2
Note that this results was also confirmed by high level ab-initio calculations which are able to correctly treat the multi-reference wave function (DOI: 10.1021/acs.jpcc.8b03935).
So your results may either indicate some severe DFT problems or a bit more is happening chemically than what you expect. Now I can not give you a more detailed answer since I do not have any detailed information regarding the systems you are considering and the computations you are performing. However, from what we have seen so far, DFT problems do not push your slope into the negative if performed according to the state of the art, spin has been treated properly and the binding energies were computed correctly (please check this with someone if you feel even slightly unsure!!!). This is true for all functionals we have considered in a benchmark (DOI: 10.1021/acs.jpcc.8b03935).
Assuming everything is ok, this might make a chemical origin of your results more likely. Here, you must take into account that the typical mono-nuclear mechanism suggested in the literature is severely oversimplified and very likely does not matter too much in reality. For a detailed discussion see DOIs: 10.1016/j.nanoen.2016.04.011 10.1016/j.coelec.2018.06.007 and 10.1021/acscentsci.9b00053 . So what could happen in your case? In my experience, the most common "error" when computing *OOH is the transfer of a hydrogen to a hydrogen acceptor site (A). So you would consider the reaction
*=O + H2O + *A -> *O2 + H+ + e- + *A-H
rather than
*=O + H2= -> *-OOH + H+ + e-
This reaction may, depending on how reactive A is, affect the binding energy of *-"OOH" severely and even result in a apparently negative slope. You can spot this reaction easily by visually inspecting your converged structures. Note that, in case this indeed happened, this does not indicate an error on your side but simply highlights that the reaction proceeds through a bi-functional mechanism (see DOIs: 10.1016/j.nanoen.2016.04.011 10.1016/j.coelec.2018.06.007 and 10.1021/acscentsci.9b00053). Should you, under these conditions, want to enforce an *OOH intermediate than your only hope is to let the hydrogen point away from the acceptor group and hope that the transfer has a significant barrier or to apply some suitable constraints.
Best wishes
Michael
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