  We propose a simple theoretical model which explains a formation of dipolar
2D and 3D colloidal structures in nematic liquid crystal. Colloidal particles
are treated as effective hard spheres interacting via their elastic dipole,
quadrupole and octopole moments. It is shown that octopole moment plays an
important role in the formation of 2D and 3D nematic colloidal crystals. We
generalize this assumption on the case of the external electric field and
theoretically explain a giant electrostriction effect in 3D crystals observed
recently [A. Nych et al., Nature Communications \textbf{4}, 1489 (2013)].
