Calculates the interaction Hamiltonian. The Hund coupling is a
           fraction of the coulom interaction

def inter_spin_hamiltonian(self, u_int, J_coup):
        """Calculates the interaction Hamiltonian. The Hund coupling is a
           fraction of the coulom interaction"""
        J_coup *= u_int
        h_int  = (u_int - 2*J_coup)/2.*self.oper['sumSz2']
        h_int += J_coup*self.oper['sumSz-sp2']
        h_int -= J_coup/2.*self.oper['sumSz-or2']
        h_int -= J_coup*self.oper['Sfliphop']

        return h_int