Modeling Correlation in the Continuum – MoCoCo

Chemical processes can occur via the discrete part of the electronic spectrum, where one finds the ground and the low-lying excited states, but also via the continuum, in phenomena ranging from electron scattering to photoionization and strong-field processes. Embedded in the continuum, resonances are remarkable states where the electrons are temporarily bound until one of them escapes. There is a growing recognition that resonances play major roles in the chemistry of many environments (radiochemistry, astrochemistry, cold plasma, etc.) which urges the need for reliable theoretical modeling. While the description of both bound states and resonances have to face the major challenge of treating the electronic correlation, the case of resonances is more complex since the continuum of unbound states must be equally considered. Because of this additional layer of difficulty, current methods targeting molecular resonances are less accurate than bound-state methods. Modeling Correlation in the Continuum (MoCoCo) aims at developing highly-accurate theoretical methodologies for resonances. To reach this goal, MoCoCo will couple selected configuration interaction, which can systematically capture correlation effects, with two different frameworks to handle the continuum, namely, (i) the method of complex basis functions, and (ii) the Schwinger variational method. The former relies on a bound-state formalism to locate the resonance, while accounting for the continuum implicitly. The latter is based on scattering theory, where the resonance naturally emerges within the explicitly described continuum. By exploiting the advantages of both implementations, MoCoCo will provide new routes to efficiently and accurately target resonances.