First-principle interaction potentials for metastable He(³S) and Ne(³P) with closed-shell molecules. Application to Penning-Ionizing systems

We present new interaction potential curves, calculated from first-principle, for the He$(^3$S)$...$H$_2$ and He$(^3$S)$...$Ar systems, relevant in Penning's ionization experiments. Two different approaches were applied: supermolecular using coupled cluster theory (CC) and perturbational within symmetry-adapted perturbation theory (SAPT). Both methods gave consistent results and the potentials were used to determine the positions of shape resonances in low collision energy scattering regime. We found a good agreement with the most recent scattering experiment of Henson {\em et al.} [Science {\bf 338}, 234 (2012)]. In addition, we investigated two other dimers, composed of metastable Ne and ground state He and Ar atoms. For the Ne$(^3$P)$...$He system a good agreement between CC and SAPT approaches was obtained. The Ne$(^3$P)$...$Ar dimer was described only with SAPT, as CC gave divergent results. Ne$^*$ systems exhibit extremely small electronic orbital angular momentum anisotropy of the potentials. We attribute this effect to the screening of the open 2$p$ shell by the singly occupied 3$s$ shell.