Internal rotation overall - rotation distortion operator in XIAM_mod:

  Hird =  2 Dpi2J (p_alpha - rho P_r)**2 P**2
         + Dpi2K [(p_alpha - rho P_r)**2 P_z**2 
                                  + P_z**2 (p_alpha - rho P_r)**2]
         + Dpi2- [(p_alpha - rho P_r)**2 (P_x**2 - P_y**2)
                                  + (P_x**2 - P_y**2) (p_alpha - rho P_r)**2]
         + Dc3J  cos(3alpha) P**2
         + Dc3K  cos(3alpha) P_z**2         
         + Dc3-  cos(3alpha) (P_x**2 - P_y**2)