In this paper we propose a possible explanation of the fact that certain molecules exhibiting symmetric configurations are localized in one of them, instead of being delocalized as predicted by a naive application of quantum mechanics. Our mechanism is based upon the instability, in the semiclassical regime, of delocalized states under small perturbations (among which we consider the reaction field due to the environment). This mechanism predicts, for example, a different behavior for ${\mathrm{NH}}_3$ and ${\mathrm{AsH}}_3$ (delocalized ground state in the former case, localized ground state in the latter case), and similar to the case of ${\mathrm{AsH}}_3$ we expect localized states in the case of optical isomers, in agreement with the empirical evidence. Possible consequences concerning the understanding of the ‘‘classical limit’’ in general are briefly outlined.

• Received 3 September 1985

DOI:https://doi.org/10.1103/PhysRevA.33.2245