Sodium ions Na⁺ and fluorine ions F^- have almost the same ionic radius.  Both have small polarizabilities.  Stoichiometric clusters Na_NF_N are known to adopt, when possible, "cuboidal" geometries which resemble fragments of the parent rocksalt structure material NaF.  For these reasons, it is an ideal system for theoretical investigation of electrical polarity and how it evolves from small clusters toward bulk crystals.

There are also very interesting clusters Na_N+1F_N which are slightly off stoichiometry.  These may also find simple cuboidal structures.  In particular, Na₁₄F₁₃ is a 3 x 3 x 3 cuboid with cubic (O_h) point symmetry.  It has one extra electron outside the closed shell system of sodium and fluorine ions.  This last electron is particularly interesting, and makes the molecular cluster highly polarizable.  The highest occupied electronic orbital (the "homo" level) looks like this.
  Notice that the wavefunction is concentrated on the 8 corner Na atoms.  The next level up has threefold orbital degeneracy.  It is the Tiu or vector representation of the Oh point group.  These functions, exactly like ordinary atomic px, py, pz functions, have nodal planes perpendicular to the x, y, and z directions.