  We have performed systematic large-scale all-electron correlated calculations
on boron clusters B$_{n}$(n=2--5), to study their linear optical absorption
spectra. Several possible isomers of each cluster were considered, and their
geometries were optimized at the coupled-cluster singles doubles (CCSD) level
of theory. Using the optimized ground-state geometries, excited states of
different clusters were computed using the multi-reference singles-doubles
configuration-interaction (MRSDCI) approach, which includes electron
correlation effects at a sophisticated level. These CI wave functions were used
to compute the transition dipole matrix elements connecting the ground and
various excited states of different clusters, eventually leading to their
linear absorption spectra. The convergence of our results with respect to the
basis sets, and the size of the CI expansion were carefully examined. The
contribution of configurations to many body wavefunction of various excited
states suggests that the excitations involved are collective, plasmonic type.

Atomic and Molecular Clusters;Materials Science