A potential energy curve (PEC) accurate to a fraction of 1 ppm (1:106) is computed for the a 3Σu+ state of He2 endowed with relativistic and QED corrections. The nuclear Schrödinger equation is solved on this PEC with diagonal Born-Oppenheimer and nonadiabatic mass corrections to obtain highly accurate rotational-vibrational levels. The computed rovibrational intervals and fine-structure splittings, spanning over several orders of magnitude in energy, are found to be in remarkable agreement with available high-resolution spectroscopy data.
Margócsy et al. (2026) studied this question.