Low-symmetry nuclear displacement

In the OOA, as it was discussed above, ligands are omitted. The vibrational motion of low-symmetry distortions is averaged out. Electron wave functions do not follow nuclear displacements. Therefore, in the OOA, the abovementioned crystal-lattice mechanism of spontaneous polarization is lost. The only possibility left in the OOA is polarization of electron wave functions. Without lattice distortion involved, this pure electron-shell mechanism was discussed in literature long ago, back about 45 years. Lacking supporting evidence from experimental data and electron-structure evaluation, it was rejected. On the contrary, the vibronic theory of ferroelectricity cumulated overwhelming experimental evidence. (For an updated review, see Sect. 8.3 in Bersuker s book [1].)... 

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