I am currently studying diffusion of oxygen in high entropy alloys. I want to calculate the activation energy from an adsorption site on the surface to the subsurface. I placed the oxygen atom below the adsoprtion site which is the octahedral site. However, the energy there is high and during the transition state calculation there is no transition state even when at the point where the oxygen atom lies on the same plane with the surrounding atoms. Any idea why? For instance the energy I got from one adsorption site is -703.9264 eV whereas that for the final state is -701.6669 eV. I have attached my two POSCARS for the final and initial states.