Ab initio model potential embedded cluster calculations including lattice relaxation and polarization: Local distortions on Mn2+-doped CaF2

Abstract

The ab initio model potentials initially developed as effective core potentials, have been proposed as embedding potentials in the field of embedded cluster calculations on impurities in ionic crystals [J. Chem. Phys. 89, 5739 (1988)] and, since then, efficiently used in the theoretical study of bulk and surface problems. These potentials bring into an ab initio cluster calculation, classical and quantum mechanical interactions with a frozen crystalline environment (Madelung, short-range Coulomb, exchange, and orthogonality) at a reasonable cost. In this paper, we extend the ab initio model potential embedding method in order to include the effects of dipole polarization and site relaxation of lattice ions external to the cluster, which are represented by an empirical shell model. We apply the method to the ab initio calculation of local distortions around a Mn2+ impurity in CaF2 lattice in the ground state (6A1g) and two excited states (4T1g,4A1g) of the cubic MnF6−8 embedded cluster. In this material, x-ray-absorption near-edge structure and extended x-ray-absorption fine-structure measurements exist which provide quantitative experimental values of first and second coordination shell distortions around Mn2+ in its ground state; the comparison with these experiments is very satisfactory. The calculated distortions in the excited states are indirectly validated by calculations of absorption/emission transitions.