  First principles density functional calculations within the local spin
density approximation (LSDA) and the generalized gradient approximation (GGA)
are reported for pyrochlore Cd$_2$Os$_2$O$_7$ and Cd$_{2}$Re$_{2}$O$_7$. The
transition metal $t_{2g}$ manifolds are found to be well separated in energy
from the O $2p$ bands and from the higher lying $e_g$ and Cd derived bands. The
active electronic structure in the $t_{2g}$ manifold near the Fermi energy,
$E_F$ is found to be significantly modified by spin orbit interactions, which
we include. Both materials show semi-metallic band structures, in which the
$E_F$ lies in an pseudogap. The band structure of Cd$_2$Os$_2$O$_7$ near $E_F$
is dominated by very heavy hole and electron bands, though at $E_F$ the
electron sections are lighter. Cd$_{2}$Re$_{2}$O$_7$ has heavy hole bands but
moderate mass electron states. The results are discussed in terms of measured
transport and thermodynamic properties of these compounds as well as the very
different ground states of these two materials.
