  We investigate when the energy that pins a superfluid vortex to the lattice
of nuclei in the inner crust of neutron stars can be approximated by the energy
that binds the vortex to a single nucleus. Indeed, although the pinning energy
is the quantity relevant to the theory of pulsar glitches, so far full quantum
calculations have been possible only for the binding energy. Physically, the
presence of nearby nuclei can be neglected if the lattice is dilute, namely
with nuclei sufficiently distant from each other. We find that the dilute limit
is reached only for quite large Wigner-Seitz cells, with radii > 55 fm; these
are found only in the outermost low-density regions of the inner crust. We
conclude that present quantum calculations do not correspond to the pinning
energies in almost the entire inner crust and thus their results are not
predictive for the theory of glitches.
