Efficient strategies for accurate calculations of electronic excitation and ionization energies: theory and application to the dehydro-m-xylylene anion.

In the dehydro-m-xylylene (DMX) anion [Munsch; et al. J. Org. Chem. 2004, 69, 5735], three nearly degenerate orbitals host four electrons, which results in a large number of nearly-degenerate electronic states. By using this challenging example, we assess the performance of the multireference "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.