Crossover between ionic/covalent and pure ionic bonding in magnesium oxyde clusters

An empirical potential with fluctuating charges is proposed for modeling $(\mathrm{MgO}{)}_{n}$ clusters in both the molecular (small $n)$ and the bulk $(\stackrel{\ensuremath{\rightarrow}}{n}\ensuremath{\infty})$ regimes. Vectorial polarization forces are explicitly taken into account in the self-consistent determination of charges. Our model predicts cuboid cluster structures, in agreement with previous experimental and theoretical results. The effective charge transferred between magnesium and oxygen smoothly increases from one to two, with an estimated crossover size above 300 MgO molecules.