Fig. qualitatively assess whether the reaction energy barrier, Ea, is also defined by εd, like the molecular and atomic Eads as found above. The plot shows how fcc has a different behaviour when compared to hcp and bcc, and the better linearity for close-packed crystallographic phases than for bcc TMs. Figs. 9, 11, and 12 illustrate that, in general, εd defines the reactants and products adsorption energies, and these, in turn, define the reaction energy difference, ΔE. The reaction energy also conditions the extent of the reaction energy barrier, Ea, aligned with the Brønsted-Evans-Polanyi (BEP) relationships .