  Recently, Vito Diana, Andrea Bacigalupo, Marco Lepidi and Luigi Gambarotta
proposed an anisotropic continuum-molecular model based on pair potentials for
describing the linear behavior of periodic heterogeneous Cauchy materials in
the framework of peridynamic theory. Unfortunately, the proposed model for an
isotropic material may have zero or even negative shear stiffness and this will
be the case for Silicene or Germanene, for example. In addition, 2D symmetries
were confused with 3D symmetries, and hence the limits for the Poisson's ratio.
