Optical absorption in boron clusters B6 and B 6 + : 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 geometries of several low-lying isomers of these clusters were optimized at the coupled-cluster singles doubles (CCSD) level of theory. With these optimized ground-state geometries, excited states of different isomers were computed using the configuration-interaction singles (CIS) 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.