Abstract: This paper systematically examines the patterns and common misconceptions associated with the estimation of the HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) energy gap in compounds using spectroscopic and electrochemical methods. The HOMO-LUMO gap, as a theoretical concept, cannot be directly obtained through experimental measurements. Commonly employed techniques, such as UV-Vis spectroscopy and cyclic voltammetry, yield data that serve as approximations of the optical gap or the fundamental gap, both of which are fundamentally distinct from the true HOMO-LUMO gap. This analysis explores the theoretical foundations and inherent limitations of these methods, highlighting the significant influence of factors such as exciton binding energy, orbital relaxation, and solvent effects on the measured values. It emphasizes the lack of rigor in directly equating experimental data with the HOMO-LUMO gap and calls for a precise understanding of the distinct physical meanings of these different energy gaps in both research and educational contexts, thereby enhancing the scientific accuracy and reliability of data interpretation.

Key words: HOMO-LUMO gap, Optical gap, Fundamental gap, UV-Vis spectroscopy, Cyclic voltammetry