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.