Limiting properties of (MgO)n and (CaO)n clusters

The limiting behaviours of bond distances and binding energies are studied for small alkaline earth oxide clusters, employing the B3LYP density functional. Validation of the applicability of density functional theory is made on selected clusters by MP4 calculations. Basis set effects on cluster structures and binding energies are determined, and comparisons between (MgO)(1-16) and (CaO)(1-12) are made. For CaO, the need to include at least two d-polarisation functions for a proper description of the cluster binding energy is emphasised. Consistency in limiting binding energies for monolayers of MgO and CaO, determined here to be 8.3 eV and 9.2 eV respectively, in conjunction with MP4 calculations on (MgO)(4), is employed to critically reassess claimed converged bulk binding energies for small cube-like MgO clusters.