We present a method for calculating the spectrum of periodic solids withinreduced density matrix functional theory. This method is validated by adetailed comparison of the angular momentum projected spectral density withthat of well established many-body techniques, in all cases finding anexcellent agreement. The physics behind the pressure induced insulator-metalphase transition in MnO is investigated. The driving mechanism of thistransition is identified as increased crystal field splitting with pressure,resulting in a charge redistribution between the Mn $e_g$ and $t_2g$ symmetryprojected states.
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