For Mg i , the +2 charge state corresponds to the depopulation of the bonding a 1g molecular orbital (Figure ), which exists in the conduction band so that Mg +2 i forms when E F is in the band gap. The +2 charge state of Si i suggests that the fully-occupied singlet bonding a 1g state is in the valence band and the partiallyoccupied triplet bonding t 1u states are in the conduction band (Figure ), such that the charge state can be interpreted as a depopulation of the t 1u states by 2 electrons.