  We theoretically propose the necessary conditions for realization of giant
Rashba splitting in bulk systems. In addition to (i) the large atomic
spin-orbit interaction in an inversion-asymmetric system, the following two
conditions are further required; (ii) a narrow band gap, and (iii) the presence
of top valence and bottom conduction bands of symmetrically the same character.
As a representative example, using the first principles calculations, the
recently discovered giant bulk Rashba splitting system BiTeI is shown to fully
fulfill all these three conditions. Of particular importance, by predicting the
correct crystal structure of BiTeI, different from what has been believed thus
far, the third criterion is demonstrated to be met by a negative crystal field
splitting of the top valence bands.
