  A series of calculations on MgB2 and related isoelectronic systems indicates
that the layer of Mg(2+) ions lowers the non-bonding B "pi" (p_z) bands
relative to the bonding "sigma" (sp_xp_y) bands compared to graphite, causing
sigma --> pi charge transfer and sigma band doping of 0.13 holes/cell. Due to
their two dimensionality the sigma bands contribute strongly to the Fermi level
density of states. Calculated deformation potentials of Gamma point phonons
identify the B bond stretching modes as dominating the electron-phonon
coupling. Superconductivity driven by sigma band holes is consistent with the
report of destruction of superconductivity by doping with Al.
