  A study is presented of the localisation of excitonic states on extended
molecular aggregates composed of identical monomers arising, not from disorder
due to statistical energy shifts of the monomers, induced by environmental
interactions (Anderson localisation), but rather simply due to changes in the
orientation and geometrical arrangement of the transition dipoles. It is shown
further that such small changes nevertheless can have a drastic effect on the
shape of the vibronic spectrum of the aggregate. The vibronic spectra are
calculated using the "coherent exciton scattering" (CES) approximation whose
derivation we generalise to be applicable to aggregates of arbitrary size and
geometry.
