Spin Hamilton Operators, Symmetry Breaking, Energy Level Crossing and   Entanglement
  We study finite-dimensional product Hilbert spaces, coupled spin systems, entanglement and energy level crossing. The Hamilton operators are based on the Pauli group. We show that swapping the interacting term can lead from unentangled eigenstates to entangled eigenstates and from an energy spectrum with energy level crossing to avoided energy level crossing. 
Willi-Hans Steeb, Yorick Hardy and Jacqueline de Greef
quant-ph