defect physics. The interstitial atoms are a part of the host crystals, in a sense that the energy is lower than that of the perfect crystals. The deviation from stoichiometry in $\beta$-R boron led Ogitsu and Widom to conceive a new concept of [*geometrical frustration*]{}.[@Ogitsu09; @Ogitsu10; @Widom08] Shirai and Uemura have finally established a relationship between the deviation from stoichiometry and the metal/insulator problem.

The mechanism behind the insulating property of boron crystals, which are otherwise predicted as metals, can be elucidated by the following way.[@Shirai12] (i) The system has an odd number of electrons ($N_{\rm el}$), which is a consequence of electron deficiency. Then band theory unequivocally predicts it to be a metal. (ii) The system has strong covalency. This requires an even number of electrons. These two conditions (i) and (ii) are mutually incompatible. This competition yields a strong driving force to modify chemical bonds. Reconstruction of chemical bonds is usually difficult because of high energy barriers which exist between different configurations. By combining with the third condition (iii) of a large unit cell size (large number of atoms $N_{\rm at}$ per cell), the energy barrier can largely be reduced. The deviation from stoichiometry is an efficient solution 