  We measure the adsorption height of hydrogen-intercalated quasi-free-standing
monolayer graphene on the (0001) face of 6H silicon carbide by the normal
incidence x-ray standing wave technique. A density functional calculation for
the full ($6 \sqrt{3} \times 6 \sqrt{3}$)-R30$^\circ$ unit cell, based on a van
der Waals corrected exchange correlation functional, finds a purely
physisorptive adsorption height in excellent agreement with experiments, a very
low buckling of the graphene layer, a very homogeneous electron density at the
interface and the lowest known adsorption energy per atom for graphene on any
substrate. A structural comparison to other graphenes suggests that hydrogen
intercalated graphene on 6H-SiC(0001) approaches ideal graphene.
