  In this work density functional theory (DFT) and diffusion Monte Carlo (DMC)
methods are used to calculate the binding energy of a H atom chemisorbed on the
graphene surface. The Perdew-Burke-Ernzerhof (PBE) value of the binding energy
is about 20% larger in magnitude than the diffusion Monte Carlo result. The
inclusion of exact exchange through the use of the Heyd-Scuseria-Ernzerhof
(HSE) functional brings the DFT value of the binding energy closer in line with
the DMC result. It is also found that there are significant differences in the
charge distributions determined using PBE and DMC approaches.
