  Electronic charge delocalization on the molecular backbones of ionic
liquid-forming ions substantially impacts their molecular polarizabilities.
Density functional theory calculations of polarizabilities and volumes of many
cations and anions are reported and applied to yield refractive indices of 1216
ionic liquids. A novel expression for the precise estimation of the molecular
volumes of the ionic liquids from simulation data is also introduced, adding
quadratic corrections to the usual sum of atomic volumes. Our significant
findings include i) that the usual assumption of uniform, additive atomic
polarizabilities is challenged when highly mobile electrons in conjugated
systems are present, and ii) that cations with conjugated large carbon chains
can be used together with anions for the design of ionic liquids with very high
refractive indices. A novel relation for the polarizability volume is reported
together with a refractive index map made up of the studied ionic liquids
