  We report a first-principles density-functional calculation of the electronic
structure and properties of the recently discovered superconducting
beta-pyrochlore oxide KOs_2O_6. We find that the electronic structure near the
Fermi energy E_F is dominated by strongly hybridized Os-5d and O-2p states. A
van Hove singularity very close to E_F leads to a relatively large density of
states at E_F, and the Fermi surface exhibits strong nesting along several
directions. These features could provide the scattering processes leading to
the observed anomalous temperature dependence of the resistivity and to the
rather large specific heat mass enhancement we obtain from the calculated
density of states and the observed specific heat coefficient. An estimate of
T_c within the framework of the BCS theory of superconductivity taking into
account the possible effects of spin fluctuations arising from nesting yields
the experimental value.
