The dynamics of the NH 4 rotor orientation in perovskites NH 4MnX 3 with X = F or Cl, during their crystallographic transition with temperature, was studied by the van der Waals energy minimization between NH 4 and the perovskite cubo-octahedral X 12 cage around it. It is concluded that, at temperatures above T c , NH 4 has an almost free rotation, although in some deformed crystal domains a specific NH 4 rotor orientation with four N-H .. X hydrogen bonds is preferred. At T < T c these NH 4 orientations are associated with the condensation of soft mode lattice vibrations to produce the same type of deformation in the whole crystal.