Pressure dependence of the energy spectrum of trigonal Se

The pressure dependence of the energy spectrum (bands, density of states (DOS) and core levels) of trigonal Se up to 140 kbar is calculated within the X alpha scheme ( alpha =1 and 2 / 3 ) using the self-consistent symmetrised orthogonalised-plane-wave (SCSOPW) method. The minimum optical gap at zero pressure is from M 2 to H 1 and the only pressure coefficient ( delta E g / delta P) P=0 agreeing with experiments is associated with the same transition. The valence band maximum shifts at 33 (23) kbar for alpha =1( 2 / 3 ) from M 2 to H 3 and the pressure coefficient doubles, which unambiguously explains the anomaly found to the experimental absorption edge at 35 kbar. At zero pressure the SCSOPW DOS and the core level spectrum agree well with the X-ray photoemission results. The pressure-dependent behaviour of the energy spectrum is presented and analysed.