Quantum-chemical simulation of impurity-induced trapping of a hole: [li]° centre in mgo

We present and describe a new embedded-cluster approach for calculating both the electronic and the spatial structure of point defects in ionic crystals with self-consistent incorporation of the polarisation effects into the eigenvalue spectrum and the total energy of a crystal. For the first time both the electronic structure and the ionic displacements within a large cluster containing a defect are obtained in the framework of the same calculating scheme. It permits us to describe correctly back-coupling between redistribution of the electron density in a defective region and polarisation of the rest of the crystal outside the cluster, which can play a vital role for the study of small polaron effects. Making use of a 45- atom cluster with 175 electrons and the semi-empirical indo method, we have simulated the impurity-induced trapping of a hole in the [Li]^o centre in an MgO crystal. Our calculations do not use a priori assumptions and do confirm that one-centre localisation of a hole in the ground state is energetically favourable. We show that polarisation energies can differ considerably for the ground and charge-transfer excited states of the defect. It is also demonstrated that the generally accepted model of the O^- hole polaron is oversimplified: A considerable charge redistribution between Li⁺ and another O^- along the (100) direction takes place.