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. To calculate rovibronic resonances,
the pre-Born–Oppenheimer variational approach of [J.
Chem. Phys. 2012, 137, 024104],
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,
Ps2. Furthermore, the excited bound and resonance rovibronic
states of the four-particle H2 molecule are also considered.
Resonance energies and widths are estimated for the lowest-energy
resonances of H2 beyond the b 3∑u+ continuum.