  The origin of ferroelectricity in KH_2PO_4 (KDP) is studied by
first-principles electronic structure calculations. In the low-temperature
phase, the collective off-center ordering of the protons is accompanied by an
electronic charge delocalization from the "near" and localization at the "far"
oxygen within the O-H...O bonds. Electrostatic forces, then, push the K+ ions
towards off-center positions, and induce a macroscopic polarization. The
analysis of the correlation between different geometrical and electronic
quantities, in connection with experimental data, supports the idea that the
role of tunnelling in isotopic effects is irrelevant. Instead, geometrical
quantum effects appear to play a central role.
