  We present systematic and comprehensive correlated-electron calculations of
the linear photoabsorption spectra of small neutral closed- and open-shell
sodium clusters (Na_{n}, n=2-6), as well as closed-shell cation clusters
(Na_{n}^{+}, n=3, 5). We have employed the configuration interaction (CI)
methodology at the full CI (FCI) and quadruple CI (QCI) levels to compute the
ground, and the low-lying excited states of the clusters. For most clusters,
besides the minimum energy structures, we also consider their energetically
close isomers. The photoabsorption spectra were computed under the
electric-dipole approximation, employing the dipole-matrix elements connecting
the ground state with the excited states of each isomer. Our calculations were
tested rigorously for convergence with respect to the basis set, as well as
with respect to the size of the active orbital space employed in the CI
calculations. These calculations reveal that as far as electron-correlation
effects are concerned, core excitations play an important role in determining
the optimized ground state geometries of various clusters, thereby requiring
all-electron correlated calculations. But, when it comes to low-lying optical
excitations, only valence electron correlation effects play an important role,
and excellent agreement with the experimental results is obtained within the
frozen-core approximation. For the case of Na_{6}, the largest cluster studied
in this work, we also discuss the possibility of occurrence of plasmonic
resonance in the optical absorption spectrum.
