Disentangling phase transitions and critical points in the   proton-neutron interacting boson model by catastrophe theory
  We introduce the basic concepts of catastrophe theory needed to derive analytically the phase diagram of the proton-neutron interacting boson model (IBM-2). Previous studies [1,2,3] were based on numerical solutions. We here explain the whole IBM-2 phase diagram including the precise order of the phase transitions in terms of the cusp catastrophe. 
J.E. Garcia-Ramos, J.M. Arias, J. Dukelsky
nucl-th