Negative Electron Affinities from DFT: Fluorination of Ethylene

Abstract

Four simple density functional theory methods are investigated to assess how well they can describe subtle variations in negative vertical electron affinities arising due to the successive fluorination of ethylene, where the magnitude of the variations is well below the absolute accuracy of the methods. Three of the approaches (two based on Koopmans' theorem and one using a potential wall) are able to describe the trends in the affinities using both compact and very diffuse basis sets. The best results are obtained using the potential wall. The fourth approach uses a conventional energy difference evaluation, which is only applicable when the basis set is compact; the breakdown upon addition of diffuse functions is strikingly illustrated. The results are interpreted in terms of the apatial extent of the relevant orbitals and the signs and values of the orbital energies. The study highlights the potential predictive value of simple DFT methods for describing trends in negative affinities.