Pyramidal Atoms: Berylliumlike Hollow States
  Based on the idea that four excited electrons arrange themselves around the nucleus in the corners of a pyramid in order to minimize their mutual repulsion, we present an analytical model of quadruply excited states. The model shows excellent comparison with ab initio results and provides a clear physical picture of the intrinsic motion of the four electrons. The model is used to predict configuration-mixing fractions and spectra of these highly correlated states. 
Marianne Dahlerup Poulsen and Lars Bojer Madsen