Density Functional Theory with Complex Absorbing Potentials: A Fast and Accurate Way of Modeling Metastable Anions

The use of a complex absorbing potential (CAP) for the description of temporary anions is well established. However, its combination with Kohn-Sham density functional theory (DFT) has so far been limited to the local density approximation. We report an implementation of CAP-DFT and complex-variable density functional approximations up to the generalized gradient approximation and derived hybrid functionals, which enables fast and accurate evaluation of the energies and lifetimes of metastable molecular anions. This new method is applied to various molecular systems, including the metastable anions of molecular nitrogen, formaldehyde, formic acid, ethene, and pyrene. Our results indicate that pure density functional approximations place temporary anions too low in energy and overestimate their decay width, while Hartree-Fock theory shows the opposite trends. Hybrid functionals balance these trends and deliver results that are competitive with equation-of-motion coupled-cluster theory.