Which NICS Aromaticity Index for Planar π Rings Is Best?

Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chemical shifts (NICS), were evaluated against a uniform set of aromatic stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)_π<i>_zz</i> (based on the π contribution to the out-of-plane <i>zz</i> tensor component), performed best statistically (cc = 0.980) and in practice. The easily computable NICS(1)<i>_zz</i> index is a useful alternative (cc = 0.968).