  The mechanism to realize the peculiar flat bands generally existing in RCo5
(R=rare earth) compounds is clarified by analyzing the first-principles band
structures and the tight-binding model. These flat bands are constructed from
the localized eigenstates, the existence of which is guaranteed by the partial
cancelation between the intersite hopping amplitudes among the Co-3d states at
the Kagome sites and those between the Kagome and honeycomb sites. Their
relative positions to other bands can be controlled by varying the lattice
parameters keeping their dispersion almost flat, which suggests the possibility
of flat-band engineering.
