Optical absorption in boron clusters B6 and B6+: a first principles configuration interaction singles approach

The linear optical absorption spectra in neutral boron cluster B6 and cationic B6 + are calculated using a first principles correlated electron approach. The many-body wavefunctions of various excited states have been analyzed and the nature of optical excitation involved are found to be of collective, plasmonic type. We also benchmark our CIS results against more sophisticated equation-of-motion (EOM) CCSD approach for a few isomers.