Comparative study of methodologies for calculating metastable states of small to medium-sized molecules.

We present a benchmarking study on the performance of two methods at the forefront of studying electronic metastable states of molecules: the orbital stabilization method and the method of complex absorbing potential augmented Hamiltonians. The performance of the two methods is compared for the calculation of shape resonances in small to medium-sized molecules (up to 15 atoms) at the equation of motion coupled cluster with singles and doubles for the electron attachment level of methodology using even-tempered Gaussian basis sets. The theoretical positions and widths of shape resonances obtained from both methods are compared to the experimentally determined electron affinities and lifetimes. The challenges that accompany the theoretical estimation of resonance positions and widths for medium to large-sized systems with an increase in basis set size are also discussed.