  We report diffusion Monte Carlo calculations on the phase diagrams of para-H2
and ortho-D2 adsorbed on top of a first layer of the same substances on
graphene. We found that the ground state of the second layer is a triangular
incommensurate solid for both isotopes. The densities for promotion to a second
layer and for the onset of a two-dimensional solid on that second layer compare
favorably to available experimental data in both cases.
