  Based on the first-principles calculations, we have investigated the
geometry, binding properties, density of states and band structures of the
novel superconductor LaFe1-xCoxAsO and its parent compounds with the ZrCuSiAs
structure. We demonstrate that La-O bond and TM-As (TM=Fe or Co) bond are both
strongly covalent, while the LaO and TMAs layers have an almost ionic
interaction through the Bader charge analysis. Partial substitution of iron
with cobalt modify the Fermi level from a steep edge to a flat slope, which
explains why in this system Co doping suppresses the spin density wave (SDW)
transition.
