Spin-selective electronic softening vs. rigidity: distinct stabilisation mechanisms in alkali- and alkaline-earth-doped MgH2 clusters.

We present a density functional theory investigation of pristine and doped (MgH2)n clusters (n = 2-4) as model systems that maximise nanoscale confinement effects, focusing on the distinct electronic regimes introduced by alkali (Li, Na, and K) and alkaline-earth (Be and Ca) dopants. The results reveal that monovalent substitution triggers a spin-selective electronic softening, with chemical hardness approaching zero in the minority spin channel accompanied by hydrogen-centred spin localisation and the emergence of weakly hydridic hydrogen sites. In contrast, divalent dopants preserve electronic rigidity and spin symmetry, maintaining the ionic coherence of the hydride framework. This difference demonstrates that by simply choosing the dopant valence, one can switch the electronic character of MgH2 nanoclusters between magnetically active/electronically soft and non-magnetic/electronically rigid states. The ability to tune the band gap and spin-dependent chemical hardness in a lightweight, Earth-abundant nanomaterial suggests possible routes toward tunable spin-dependent electronic responses in low-dimensional hydride nanomaterials, where controlled electronic softness and magnetic response are critical.