Molecular Cluster Approach to Small‐Radius Impurity Centres in Solids

The molecular cluster approach to the problem of small‐radius impurity centres in solids is discussed. It is shown that the molecular orbitals obtained for the cluster of host atoms can be connected with the crystalline states only for clusters having the form of a large unit cell. In this case the molecular orbitals of the cluster have the sense of Wannier functions, the appropriate one‐electron energies the sense of mean values of the band energies in the large unit cell scheme of the crystalline state classification. It is found that the cluster model is realistic for solids with narrow (1 to 5 eV) energy bands. The convergence of the results with increasing cluster size is shown for lithium chloride ([LiCl 6 ] 5− [Li 14 Cl 13 ] + , and [Li 18 , Cl 18 ] 0 are considered), the molecular cluster calculations are performed for some alkali halides and the results are compared with those obtained using band structure calculations.