  For $\rm Sn_2P_2S_6$ ferroelectrics by first-principles calculations an
analysis of the $\rm P_2S_6$ cluster electronic structure and their comparison
with crystal valence band in paraelectric and ferroelectric phases has been
done and the origin of ferroelectricity has been outlined. It was established
that the spontaneous polarization follows from the stereochemical activity of
the electron lone pair of tin cations that is determined by hybridization with
$\rm P_2S_6$ molecular orbitals. Increase of the chemical bonds covalence and
their rearrangement are related to the valence band changes at transition from
paraelectric phase to ferroelectric one.
