  For the orthorhombic intermetallic semiconductor Al$_2$Ru, the bandstructure,
valence charge density, zone center optical phonon frequencies, and Born
effective charge and electronic dielectric tensors are calculated using
variational density functional perturbation theory with {\em ab initio}
pseudopotentials and a plane wave basis set. Good agreement is obtained with
recent measurements on polycrystalline samples which showed anomalously strong
far IR absorption by optical phonons, while analysis of the valence charge
density shows that the static ionic charges of Al and Ru are negligible.
Hybridization is proposed as the single origin both of the semiconducting gap
and the anomalous Born effective charges. Analogous behavior is expected in
related compounds such as NiSnZr, PbTe, skutterudites, and Al-transition-metal
quasicrystals.
