Yet another series of correlations based on harmonic oscillator model of aromaticity (HOMA).

In this work, we check the correlation between computed molecular energies and harmonic oscillator model of aromaticity (HOMA) indices, selecting representative series of isomeric polyaromatic hydrocarbons (PAHs) and derivatives. We designed numeric experiments, enforcing, by constrained geometry optimization equal bond lengths, fixed like in the aromatic reference (benzene), along the conjugated perimeter, checking correlation with HOMA. We refined the HOMA parameterization with substituent-dependent increments, based on the observation C 6 X 3 H 3 and C 6 X 6 substituted benzenes show equal bond lengths around the cycle, appearing then aromatic. The high HOMA indices in the full set of substituted benzenes show a facet of aromaticity as relative resistance to substituent effects, a point confirmed from Valence Bond calculations. The HOMA model can be improved by fine-tuned substituent terms, but this does not lead to improved correlation with energy, showing that the scheme reached its limits of interpretation. • HOMA is simplistic, by design, but can yet be polished, at conceptual level. • It is worth to regard HOMA as related with energy criteria of aromaticity. • One may propose unique bond-stretch force constants instead of unique bond length. • Cycles with D 3h symmetry can establish substituent or heteroatom parameterizations. • Valence Bond calculations offer the supreme criteria for resonance stabilization.