Field-dependent ionization potential for polyaromatic molecules from constrained density-functional theory

The field-dependent ionization potential is determined by calculating the dissociation energy barrier for the interaction between a cation and an electron in an electric field. A quantum-chemical method based on constrained density-functional theory (CDFT) has been established for this purpose. Here we present the field-dependent ionization potential and excitation energies for polyaromatic molecules relevant for electrically insulating liquids. In the CDFT model, we rely on that the dissociation barrier is located somewhere outside the cation. This causes problems for all molecules at sufficiently high electric fields, but for polyaromatic molecules the problems appear at lower fields as compared to previously studied molecules. This limitation has been investigated in detail and some initial results are presented for a set of polyaromatic molecules including benzene and pyrene. The importance of ionization potential and excitation energies in streamer initiation and propagation are discussed.