Most calculations of electronic structure in molecules use a formalism in which the electrons occupy molecular orbitals (MOs) that extend over the entire molecule. Although methods exist for transforming those delocalized MOs into more localized ones, it is generally believed that the localized picture is inappropriate for molecules with a small bandgap or HOMO/LUMO gap. Such molecules are exemplified by the polyacenes (benzene, naphthalene, anthracene, etc.) which are believed to be inherently delocalized. In an attempt to quantify the delocalization in polyacenes with up to 50 carbon atoms, we have performed self-consistent field calculations in which the electrons are constrained to occupy highly localized molecular orbitals (HILOs), each one of which is centred on a few adjacent carbon atoms. Our results lead us to conclude that the chemistry of polyacenes can be modelled well using surprisingly localized descriptions of the electrons.

Peter Gill had held academic positions in the US, NZ and the UK before moving to ANU in 2004 and thence to the University of Sydney in 2019. He has received the Dirac (1999) and Schrödinger (2011) Medals of the World Association of Theoretical and Computational Chemists (WATOC) and the Pople (2005) and Fukui (2013) Medals of the Asia-Pacific Association of Theoretical and Computational Chemists (APATCC). He is currently the president of WATOC and vice-president of APATCC. He was elected to the Australian Academy of Science in 2014 and received its Craig Medal in 2019. He founded the US software company Q-Chem in 1993 and, after leading it for 20 years, now sits on its Board of Directors.