which is specified in the same units of the frequency ω ω . A positive (negative) sign of C p h (A-B) indicates that the A (B) atom contributes more to the high frequency modes of the specified range. The smaller is | C p h (A-B) | , the higher is the mixing of the A and B contributions to the frequency band, and the two atoms have the same weight in the determination of the modes specific of the considered energy range. We define the quantity C p h (A-B) as the cophonicity of the A-B atomic pair, in analogy with the A-B bond covalency definition formulated in terms of atomic contributions to the electronic density of states.