Strong charge transfer effects in the Mg2p^{-1} core-level spectrum of   MgB_2
  The two available Mg2p^{-1} spectra of the recently discovered high-temperature superconducting MgB_2 crystal exhibit interesting structures, but contradict each other. This motivated us to perform ab initio calculations on the core-level spectra of cluster models. The computed spectra reveal unusual rich and intense structures triggered by a B2p_z->Mg3s,3p charge transfer 
N.V. Dobrodey, A.I. Streltsov, L.S. Cederbaum, C. Villani, F.   Tarantelli