Graphene on Ir(111): Physisorption with chemical modulation

The non-local van der Waals density functional (vdW-DF) approach is applied to calculate the binding of graphene to Ir(111). The precise agreement of the calculated mean height h = 3.41 {\AA} of the C atoms with their mean height h = (3.38 \pm 0.04) {\AA} as measured by the X-ray standing wave (XSW) technique provides a benchmark for the applicability of the non-local functional. We find bonding of graphene to Ir(111) to be due to the van der Waals interaction with an antibonding average contribution from chemical interaction. Despite its globally repulsive character, in certain areas of the large graphene moir\'e unit cell charge accumulation between Ir substrate and grapheneC atoms is observed, signaling a weak covalent bond formation.