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I've been running what I thought would be trivial calculations for adsorption of small molecules on the Co (without Hubbard U) and CoO (with Hubbard U) surfaces. In both cases, I get the adsorption energies which make no sense (> 4 eV).

I attach the relevant files for the calculations of clean Co surface (forumclean.zip), H2O adsorbed on the Co surface (forumadsh2o.zip), as well as the water molecule (forumh2o.zip). It can be seen that the adsorption energy is 4.2 eV, which is impossible (especially since there's only physisorption happening). The similar energies are obtained for adsorption of other molecules (like H2), leading me to believe that it's the clean surface calculation that gives the wrong energy. The error is also present on CoO surface.

I'm sorry if it's something trivial that I missed, but I just couldn't see it. Thanks!

Hi,

I would not call this calculation trivial. It is always a good idea to start with something simpler like an example on the wiki to make sure that one is able to reproduce the reference values.

In your OUTCAR files it looks like the slab with the molecule is ferromagnetic but the clean surface isn't and they have very different total magnetic moments, so I would assume that the clean slab has not been relaxed fully and has not reached the ground state. You can specify the initial magnetic moments via MAGMOM to speed up the convergence to the correct ground state.