Ab initio cluster calculations of defects in solids

A method based on local density functional theory is described which leads to the rapid determination of the structure, vibrational and electronic properties of clusters as large as 100-150 atoms. The technique is particularly suitable for molecular solids, covalently bonded materials where the clusters are terminated by hydrogen, and to ionic systems where the termination consists of a set of distributed charges. The strengths and weaknesses of the method are detailed together with an application to the interstitial carbon-oxygen complex in silicon where oxygen is found to be over-coordinated. The good agreement obtained for the vibrational modes of the complex lends support to the unusual structure found.