Insulating Oxides in Low Dimensionality A Theoretical Review

In the light of recent studies, we summarize our present theoretical understanding of insulating oxides in low dimensionality, including unsupported clusters, planar or stepped surfaces and ultrathin films. We review the various theoretical approaches, their efficiency in calculating ground and excited state properties, and their applications to the present systems. We discuss the forces at work which determine the atomic structure around under-coordinated atoms (equilibrium geometries of very small clusters, bond lengths, relaxation and rumpling at surfaces), the energetics associated with low dimensionality (surface energies and mean cohesion energy in clusters), the electronic properties, such as electron distribution, magnetic interactions and ordering, and electronic excitations (ionization potentials, electron affinity, quasiparticle spectra, d → d and charge transfer excitations).