  The geometric structure of the Rh$_8^+$ cation is investigated using a
combination of far-infrared multiple photon dissociation spectroscopy and
density functional theory (DFT) calculations. The energetic ordering of the
different structural motifs is found to depend sensitively on the choice of
pure or hybrid exchange functionals. Comparison of experimental and calculated
spectra suggests the cluster to have a close-packed, bicapped octahedral
structure, in contrast to recent predictions of a cubic structure for the
neutral cluster. Our findings demonstrate the importance of including some
exact exchange contributions in the DFT calculations, via hybrid functionals,
when applied to rhodium clusters, and cast doubt on the application of pure
functionals for late transition metal clusters in general.
