  We theoretically investigate the ground-state properties of a molecular
para-hydrogen (p-H2) film in which crystallization is energetically frustrated
by embedding sodium (Na) atoms periodically distributed in a triangular
lattice. In order to fully deal with the quantum nature of p-H2 molecules, we
employ the diffusion Monte Carlo method and realistic semi-empirical pairwise
potentials describing the interactions between H2-H2 and Na-H2 species. In
particular, we calculate the energetic, structural and superfluid properties of
two-dimensional Na-H2 systems within a narrow density interval around
equilibrium at zero temperature. In contrast to previous computational studies
considering other alkali metal species such as rubidium and potassium, we find
that the p-H2 ground-state is a liquid with a significantly large superfluid
fraction of ~30%. The appearance of p-H2 superfluid response is due to the fact
that the interactions between Na atoms and H2 molecules are less attractive
than between H2 molecules. This induces a considerable reduction of the
hydrogen density which favours the stabilization of the liquid phase.
