  Boron clusters have been widely studied theoretically for their geometrical
properties and electronic structure using a variety of methodologies. An
important cluster of boron is the B$_{12}$ cluster whose two main isomers have
distinct geometries, namely, icosahedral ($I_{h}$) and quasi planar ($C_{3v}$).
In this paper we investigate the linear optical absorption spectrum of these
two B$_{12}$ structures with the aim of examining the role of geometry on the
optical properties of clusters. The optical absorption calculations are
performed using both the semi-empirical and the ab initio approaches. The
semi-empirical approach uses a wave function methodology employing the INDO
model Hamiltonian, coupled with large-scale configuration interaction (CI)
calculations, to account for the electron-correlation effects. The \emph{ab
initio} calculations are performed within a
time-dependent-density-functional-theory (TDDFT) methodology. The results for
the two approaches are in very good qualitative agreement with each other.
Quantitatively speaking, results agree with each other in the lower energy
region, while in the higher energy region, features predicted by the TDDFT
approach are red-shifted as compared to the INDO-CI results. Both the
approaches predict that the optical absorption begins at much lower energies in
the icosahedral cluster as compared to the planar one, a fact which can be
utilized in experiments to distinguish between the two geometries. At higher
energies, both the isomers exhibit plasmon-like excitations. 73,1 4%
