Chemical indices, especially diradical character, are utilised to examine the functions and reactivity of stable radical species. In the present study, we formulated an approximation to estimate chemical indices using...

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Diradical character is important for investigating and predicting the functions of singlet biradical molecules. Singlet biradical molecules possess many unique properties, and their immobilisation by surfaces has been extensively studied for applications in devices, such as quantum computers, single-molecule magnets, single-atom catalysts, organic batteries, and organic superconductivity devices. Recently, an estimation scheme of diradical character from the calculated results of density functional theory with plane-wave basis (DFT/plane-wave), which is a general method for calculating the electronic structures of surfaces, was developed by our research group. This scheme enables us to investigate the diradical character of the open-shell structure stabilised by the surfaces. In this study, DFT/plane-wave calculations for s-indaceno [1,2,3-cd;5,6,7-c'd']diphenalene molecules, whose diradical character was experimentally observed, were performed. Using the developed scheme, it was verified whether the diradical character was modulated by the interaction with the MgO(001) surface. The calculated results showed that the diradical character increased as the distance from the surface decreased, although the enhancement effect of the surface was inhibited by the steric hindrance of the substituents.