  A transition from a cylindrical quantum dot to a highly elongated quantum
dash is theoretically studied here with an atomistic approach combining
empirical tight binding for single particle states and configuration
interaction method for excitonic properties. Large nanostructure shape
anisotropy leads to a peculiar trend of the bright exciton splitting, which at
certain point is quenched with further shape elongation, contradicting
predictions of simplified models. Moreover strong shape elongation promotes
pronounced optical activity of the dark exciton, that can reach substantial
$1\%$ fraction of the bright exciton intensity without application of any
external fields. An atomistic calculation is augmented with a elementary
phenomenological model expressed in terms of light-hole exciton add-mixture
increasing with the shape deformation. Finally, exctionic complexes $X^-$,
$X^+$, and $XX$ are studied as well and the correlations due to presence of
higher excited states are identified as a key-factor affecting excitonic
binding energies and the fine structure.
