Efficient strategies for accurate calculations of electronic excitation and ionization energies: Theory and application to the dehydro-meta-xylylene anion
In the dehydro-meta-xylylene (DMX) anion, 3 nearly degenerate orbitals host 4 electrons, which results in a large number of nearly-degenerate electronic states. By using this challenging example, we assess the performance of the multi-reference "brute force" approach and the two-step schemes based on single-reference methods for calculating accurate energy differences. Different schemes for calculating adiabatic ionization potential (IP) of DMX- are also investigated. IP calculated by single-reference based schemes is in excellent agreement with experiment.