On the Calculation of Resonances in Pre-Born-Oppenheimer Molecular Structure Theory

The main motivation for this work is the exploration of rotational-vibrational states corresponding to electronic excitations in a pre-Born-Oppenheimer quantum theory of molecules. These states are often embedded in the continuum of the lower-lying dissociation channel of the same symmetry, and thus are thought to be resonances. In order to calculate rovibronic resonances, the pre-Born-Oppenheimer variational approach of [J. Chem. Phys. 137, 024104 (2012)], based on the usage of explicitly correlated Gaussian functions and the global vector representation, is extended with the complex coordinate rotation method. The developed computer program is used to calculate resonance energies and widths for the three-particle positronium anion, Ps$^-$, and the four-particle positronium molecule, Ps$_2$. Furthermore, the excited bound and resonance rovibronic states of the four-particle H$_2$ molecule are also considered. Resonance energies and widths are estimated for the lowest-energy resonances of H$_2$ beyond the $b\ ^3\Sigma_\mathrm{u}^+$ continuum.