We performedab initiocalculations to study ferroelectric (FE) instabilities and isotope effects in the H‐bonded ferroelectric KH2PO4(KDP). We demonstrate that the source of the FE instability is the hydrogen off‐centering. This ordering, produces an electronic charge redistribution within the PO4tetrahedral units, which polarize alongc. Cluster distortions following the H off‐centered relaxation pattern in a mean‐field paraelectric (PE) phase, lead to instabilities which are significant only when the heavy ions P and K are also allowed to relax. Subsequent quantization in small clusters, leads to tunneling only for distortions including heavy ions relaxations. This explains the H double‐site occupancy observed experimentally in the PE phase, and is also in agreement with the P‐atom multi‐site distribution detected experimentally in deuterated KDP (DKDP). Mass changes due to deuteration at fixed structural parameters cannot account for the huge isotope effect. However, the main effect of deuteration is a depletion of the proton probability density at the O‐H‐O center, which in turn weakens the proton‐mediated covalency in the bridge. A lattice expansion follows then, which is coupled self‐consistently with the proton off‐centering. This self‐consistent mechanism is illustrated with a non‐linear model deduced from theab initiocalculations, and allows us to explain the huge isotope effect observed and the importance of geometrical effects proved by high‐pressure experiments. © 2003 American Institute of Physics