Theoretical and computational details
The geometries of various D 3h[MH3M]+ cations (M = Be, Mg), including those ‘capped’ by noble gas (Ng) atoms He or Ne (as represented in Figure 1), were optimized at the all-electron CCSD(T)/cc‑pVQZ level using MOLPRO.[20-21] We then checked in each case that all of the vibrational frequencies were positive, thereby confirming that we have located local minima. All of the subsequent calculations and analysis was carried out at these optimized geometries, using cc‑pVQZ basis sets throughout.