Localization of S-matrix poles in the complex energy plane using the molecular R-matrix method

Identification of resonances in computed cross sections is one of the main goals of ab initio scattering calculations. A common approach involves fitting of the data computed for real scattering energies to the Breit-Wigner form (in case of eigenphase sums) or the Lorentzian form (in case of time-delays), see e.g. [1-2]. While this approach works well for narrow resonances, it becomes unreliable especially for very wide resonances and core-excited resonances appearing in electronically inelastic calculations. In this contribution we resurrect an approach for localization of resonant S-matrix poles using the diatomic R-matrix method [3]. The exact energy factorization of the R-matrix approach is ideally suited for finding the Siegert solutions of the Schr\"{o}dinger equation in the complex plane. We describe our implementation within the polyatomic UKRmol+ codes, study its numerical properties and apply it to localization of resonances in low-energy electron collisions with pyrrole including their dependence on nuclear geometry.\\ \noindent [1] Z. Ma\v{s}\'{\i}n and J.D. Gorfinkiel, J. Chem. Phys. 137, 204312 (2012).\\ \noindent [2] D.A. Little, et al, Comp. Phys. Comm. 215, 137 (2017).\\ \noindent [3] L.A. Morgan and P.G. Burke, J. Phys. B: At. Mol. Opt. Phys. 21, 2091 (1988).