Intermolecular vibrational states are calculated for Ne...HBr,
Ne...HI, and HI(Ar)n (n=1-6) complexes using potential energy
surfaces constructed by accurate ab initio methods. Potentials
of rare gas--hydrogen halide clusters exhibit two collinear minima,
one corresponding to hydrogen lying between the heavy atoms, and the
other to hydrogen facing away from the rare gas atom. The relative
depths of the two minima are a result of a subtle balance between
polarization and dispersion interactions. Moreover, due to a large
quantum delocalization in the hydrogen bending (librational) motion
the relevance of a particular stationary point on the potential energy
surface is only limited. It is more appropriate to discuss the isomers
in terms of vibrationally averaged structures. For Ne...HBr the
potential minimum and the vibrationally averaged structure correspond
to the same isomer with hydrogen between neon and bromide. However,
for Ne...HI the global minimum corresponds to the Ne-IH
collinear geometry, while the vibrationally averaged structure has
hydrogen between the heavy atoms.  In the case of HI(Ar)n we show
that one can flip between the two isomers by adding argon atoms, which
reconciles the seemingly contradictory experimental results obtained
for the photodissociation of HI...Ar on one side, and of large
HI(Ar)n clusters on the other side.