Toy Models Reveal Intrinsic Biases in NICS: Insights to Verify NICS Interpretations

The nucleus-independent chemical shift (NICS) as an aromaticity index rests on the premise that the NICS value can be interpreted to infer those aspects of a system's current-density susceptibility ( $J$ ) that are relevant to magnetic aromaticity. While the shielding density acts as an intermediary in the inferential relation between NICS and $J$ , the complexity of shielding-density patterns in molecular systems hinders the extraction of generalizable insights. To address the complexity associated with molecular systems, toy models are introduced that isolate inherent features of the NICS- $J$ relationship and separate intrinsic NICS behavior from aromaticity. NICS is found to provide an inherently biased reflection of $J$ , resulting in artifacts that could be misinterpreted to yield conclusions that conflict with the magnetic aromaticity criterion. This work establishes a foundation for using toy models to obtain the insight needed to verify NICS interpretations with respect to the underlying $J$ in molecular systems.