In this paper, the differential effects brought about by the MgO, CaO, and SrO family of hosts on the topology of the energy surface corresponding to the ground state of the clusters ( Cu06) lo-and ( Ag06) lo-along the alg and es,, vibrational modes, are studied by means of the ab ini& embedding model potential method (AIEMP) at the restricted open-shell HartreeFock level. For all six defect crystals, the equilibrium geometries, Jahn-Teller energies, vibrational frequencies, and relative energies between D4h elongated and compressed structures (energy barriers) have been calculated using (a) two different definitions of the defect cluster, namely (MeO,)"-and (Me06M6)2+ (Me=Cu, Ag; M=Mg, Ca, Sr), (b) two different cluster LCAO expansions (including or not impurity second neighbor components), and (c) relativistic Cowan-Griffin ab inirio (core and environment) model potentials. The results show the importance of using basis set functions located at lattice sites next to the ( Cu06) lo-and LWd'"-clusters in order to mimic the results of the larger clusters (CUO~M~)~+ and b'%06&) -2+ The net relativistic effects calculated for SrO:Cu2+, Ag2+ and the individual contributions due to the impurity and to the environmental Sr2+ ions have been analyzed, all of them being rather small. The use of a simple point-charge model for the MgO, CaO, and SrO crystalline environments (Madelung potential) is shown to be inadequate and to lead to meaningless results, in agreement to what has been found in previous studies of bulk and surface defects in other ionic crystals. Given that the Ag2+ impurity in SrO, CaO, and MgO hosts is accepted to be an exceptionally clear example of transition from static to dynamic Jahn-Teller effect at low temperature, the reproduction of this trend can be considered to be a demanding test for an embedding method like the AIEMP: A joint analysis of our calculations and the available EPR experimental results is presented which reveals an agreement in the conclusion that the environmental effects promote stronger Jahn-Teller coupling from MgO to SrO. Also, the inadequacy of the strong linear Jahn-Teller coupling approximation for MgO:Cu2+ is suggested to be responsible for the present incomplete understanding of the features of its EPR spectrum, in contrast with the other systems.

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 ͑1988͔͒ 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 orthogonality͒ 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 Mn 2ϩ impurity in CaF 2 lattice in the ground state ͑ 6 A 1g ͒ and two excited states ( 4 T 1g , 4 A 1g ) of the cubic MnF 8 6Ϫ 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 Mn 2ϩ 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.

In this paper we present the results of ab initio model potential (AIMP) embedded-cluster calculations on the ground 4A2g and excited 4T2g state levels of V2+-doped KMgF3, KZnF3, KCdF3, and CsCaF3. Complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on the (VF6)4− embedded cluster. The AIMP embedding potentials represent both static and relaxed/polarized lattice effects which are found to be an important refinement due to the large local distortions produced by the V2+ impurity. The calculated local distortions are found to be considerably large, but much smaller than expected in terms of the mismatch of ionic radius of the impurity and the substituted cation. The host dependency of the crystal field splitting, which was found to violate the simple ligand field R−5-law in a wide family of V2+-doped halide crystals, if R is the metal–ligand distance in the host, is also examined and is found to be quite close to the simple ligand field theory prediction, as long as the calculated impurity-ligand distances are considered. The results of the 4A2g→4T2g absorptions are in close agreement with available experimental data. The comparatively high values of the calculated 4T2g→4A2g fluorescence indicate the need of the inclusion of intracluster Jahn–Teller coupling of the excited state. However, the host variation of fluorescence is, again, well reproduced.

Doping CeO2 with for example, Ca gives an enhanced reactivity toward reduction of SO2 by CO, and total combustion of methane. Theoretical modeling using static minimizations and molecular dynamics (MD) simulations of the doped (110) face in combination with ab initio quantum chemical cluster models shows large effects on the Ce(IV)/Ce(III) balance due to the doping. Computed oxygen-to-cerium charge-transfer energies are strongly reduced as a result of the introduction of defects and oxygen vacancies, but not sufficiently to explain the observed reactivities. The structures resulting from the MD simulations for both the doped and undoped material are in good agreement with recent experimental pulsed neutron scattering results.

An improvement in the method of production of embedding ab initio model potentials (AIMP) for embedded cluster calculations in ionic solids is proposed and applied to the oxides CeAlO 3 , CeO 2 , and UO 2 . The improvement affects the calculation of one of the AIMP components, the Pauli repulsion operator, which prevents the cluster electrons from collapsing onto the occupied orbitals of the host in embedded cluster calculations and, so, their over occupancy. The linear constants involved in such operator are proposed to be obtained in embedded cluster calculations in the perfect host, with the requirement that local structures calculated with working embedded clusters of relatively small size agree with those calculated with reference embedded clusters of much larger size.