Relativistic, quantum electrodynamics, and non-adiabatic corrections and couplings are computed for the b 3Πg and c3Σg+ electronic states of the helium dimer. The underlying Born–Oppenheimer potential energy curves are converged to 1 ppm (1: 106) relative precision using a variational explicitly correlated Gaussian approach. The quantum nuclear motion is computed over the b 3Πg–c3Σg+ (and B 1Πg–C1Σg+) 9-(12-)dimensional electronic-spin subspace coupled by non-adiabatic and relativistic (magnetic) interactions. The electron’s anomalous magnetic moment is also included; its effect is expected to be visible in high-resolution experiments. The computed rovibronic energy intervals are in excellent agreement with the available high-resolution spectroscopy data, including the rovibronic b 3Πg-state fine structure. Fine-structure splittings are also predicted for the c3Σg+ levels, which have not been fully resolved experimentally, yet.
Rácsai et al. (2025) studied this question.