Role of the self-interaction error in studying chemisorption on graphene from first-principles

Adsorption of gaseous species, and in particular of hydrogen atoms, on graphene is an important process for the chemistry of this material. At the equilibrium geometry, the H atom is covalently bonded to a carbon that puckers out from the surface plane. Nevertheless the flat graphene geometry becomes important when considering the full sticking dynamics. Here we show that GGA-DFT predicts the wrong spin state for this geometry, namely, S-z = 0 for a single H atom on graphene. We show that this is caused by fractional electron occupations in the two bands closest to the Fermi energy, an effect of the self-interaction error. It is also demonstrated that the use of hybrid functionals or the GGA+U method can be used to retrieve the correct spin solution although the latter gives an incorrect potential energy curve.