text: Consider ﬁrst a single tetrahedron with four spins 
  1, . . . , 4. The general distortions of this tetrahedron can 
  be classiﬁed by their symmetry (see for example Ref.35). 
  Only two tetragonal to orthorhombic distortion modes, 
  QE 
  2 (illustrated in FIG. 3), couple to the pseudo- 
  spins deﬁned in Section II. A complete analysis of all 
  modes is given in Appendix A. The coupling is of the 
  
  1 and QE 