A new parametrization of the semiempirical quantum-chemical INDO method is proposed for the study of the electronic structure of a wide class of perfect and defective ionic insulating crystals. The applicability of the "cut-off function" concept for the calculations of exchange interactions in the large unit cell (LUC) model is also briefly discussed. Both, the proposed calculation scheme and the parametrization are tested on a number of molecules, ionic crystals (LiH, LiF, LiCl, KCI, MgO, K2S0,), and (100)

ABSTRACT:We demonstrate that boundary effects (i.e., displacements of the cluster boundary atoms from their lattice sites and the difference between effective charges of the perfect crystal atoms and those of the cluster atoms in the case of a cluster with no point defect in it) in an embedded molecular cluster (EMC) model can be radically reduced. A new embedding scheme is proposed. It includes search for the structural elements (SE) of which perfect crystal is composed, use of corresponding to these SE expression for the total energy, and application of the degree of localization of equations consistent with the wave functions of the cluster. To get equations for the cluster wave functions, the problem of varied subsystem in the field of the frozen remaining part of the whole electron system" is investigated in the framework of a oneelectron approximation. The consideration is general for every task of this type. Homogeneous equations resulting directly from variation of the total energy expression are obtained and transformed to the eigenvalue problem equations. Orthogonality constraints are not imposed during variation. A particular case of the equations describing mutually orthogonal one-electron wave functions of the cluster staying nonorthogonal to those of the remaining crystal is found. A proposed embedding scheme is realized in the CLUSTER code based on the calculation scheme of the semiempirical INDO method. Boundary effects both in the standard (cluster in the field of the infinite lattice of nonpoint spherical charges) and new embedding scheme are investigated, calculating the clusters of LiF, MgO, NaCl, KCl, and AgCl crystals. Significant reduction of the boundary effects in the new embedding scheme is achieved. Reasons for the boundary effects are discussed.

AbskaeL Using embedded quantum cluster models, we have studied the mechanism of VK-centre 60' reorientation in KCI and UCI, and have considered the possibility that both one-centre 'dielectric' polarons and 'moleculd polarons (VK centres) mexisl in KCI. These studies have shown that lattice polarization lowers the aclivalion energy for VK reorienlalion since the VK centre slmngly polarizes near lhe barrier point. The calculated adiabatic barrier for VK reorientation is 0.63 eV, in reasonable agreemenl with the available experimental data. From the self-trapping energy calculalions and the analysis of the slruclure and mechanism of the dilfusion of the one-centre 'dielectric' polaron, it is apparent that it transforms adiabatically into a VK centre without an energy barrier.

Both the spatial and electronic structure of M$+, V; and (M$'-V;). centres (n = 1.4)inaLiFcrystal have beencalculated bysemiempirical techniquesofpairpotentials and INDO. Crystal polarization was taken into account in a self-consistent way in termsof the Mott-Littleton approach. It is shown that a consistent choice of parametrization of both methods gives rather close values for displacements of ions surrounding the centres under study. The Mgi* compensation by a cation vacancy located in its nearest surrounding along the (1 10) axis is found to be most energetically favourable. It is shown that the aggregation of impurity-vacancy dipoles can lead to the formation of a phase with a periodical structure differing From the traditional Suzuki phase.