Special Challenges When Studying Anions

Jack SimonsUniversity of Utah, Chemistry Department and Henry Eyring Center for Theoretical ChemistryCorresponding author: [email protected] Abstract: When carrying out quantum chemistry calculations on molecular anions (MAs), there are special challenges that arise, two of which are rather obvious and not difficult to address. You have to include diffuse basis functions because electron affinities (EAs) are often quite small, as a result of which the MA’s “extra” electron resides in an orbital that has a large radial extent. The fact that EAs are often small (e.g., smaller than ionization potentials and electronic excitation energies) means that electron correlation effects contribute significantly and thus should be included in the calculation. These are not the challenges I will discuss. Rather, I will focus on (1) how to effectively identify a virtual orbital of the neutral molecule into which the extra electron should be added; often, the correct orbital is not the LUMO but is buried within a “soup” of numerous LUMO+n virtual orbitals; (2) how to properly calculate the energy of the resulting anion when the energy of the anion lies above that of the neutral (i.e., when the anion is not an electronically stable species but is metastable); and (3) how to calculate the lifetime of the metastable anion using tools, some of which are already available in commonly used software packages and others that have been made available but require a bit more effort. Keywords: anions, virtual orbitals, metastable species, lifetimes, stabilization, extrapolation, complex absorbing potential. Suggested Reading: J. Simons, Molecular Anions, J. Phys. Chem. A 112, 6401-6511 (2008). This 100+ page article offers a good overview of theoretical and experimental studies of anions. Simons' Publications – Here you can find all of my publications, many of which are about anions. Nestmann B. M.; Peyerimhoff, S. CI method for determining the location and width of resonances in electron-molecule collision processes, J. Phys. B: At. Mol. Phys. 1985, 18, 4309-4319. Here is an early example of the stabilization method. Iwona Anusiewicz, Piotr Skurski, and Jack Simons, Finding Valence Antibonding Levels while Avoiding Rydberg, Pseudo-continuum, and Dipole-Bound Orbitals, J. Am. Chem. Soc. (2022) – Here you will see much of what I will be discussing. Thodika, M.; Fennimore, M.; Karsili, T. N. V.; Matsika, S., Comparative study of methodologies for calculating metastable states of small to medium-sized molecules, J. Chem. Phys., 2019, 151, 244104 – Here you will see nice examples of the extrapolation and CAP methods. Chao, J. S.-Y.; Falcetta, M. F.; Jordan, K. D., Application of the Stabilization Method to the N₂⁻(X²Πg) and Mg⁻(¹²P) Temporary Anion States, J. Chem. Phys., 1990, 93, 1125−1135 – Here you will see good examples of modern stabilization methods. Jagau, T-C.; Bravaya, K. B.; Krylov, A. I., Extending Quantum Chemistry of Bound States to Electronic Resonances, Annu. Rev. Phys. Chem., 2017, 68, 525-553. This describes the CAP method.