  The pedagogic two stste system of the ammonia molecule is used to illustrate
the phenomenon of environment induced molecular localization in pyramidal
molecules. A semiclassical model is used to describe a gas of pyramidal
molecules interacting via hard ball collisions. This modifies the tunnelling
dynamics between the classical equilibrium configurations of an isolated
molecule. For sufficiently high pressures, the model explains molecular
localization in these classical configurations. The decrease in the inversion
line frequency of ammonia, noted upon increase in pressure, is also studied.
