A DENSITY-FUNCTIONAL LCAO EMBEDDED-CLUSTER CALCULATION ON THE JAHN-TELLER DISTORTED STATES OF H2- IN CAO CRYSTAL

To address recent electron paramagnetic resonance experiments showing a distortion at the H2- Site lowering the defect symmetry from cubic t o tetragonal in CaO crystal, we have investigated the defect-induced l attice relaxation of an H2- ion in CaO with local O-h and D-4h symmetr y respectively using a first-principles embedded-cluster approach base d on the local density approximation of density functional theory. We show that the ground state in O-h symmetry is an orbitally degenerate t(1u) state, while in D-4h symmetry it is an a(1) state. This Jahn-Tel ler distortion of the CaO:H2- system is consistent with the experiment al findings. We find that the position of the HOMO t(1u) root in the b andgap is sensitive to the inclusion of the 3d orbital at the nearest- neighbour Ca2+ sites suggesting a bonding interaction. The Fermi conta ct spin density at the proton is in good agreement with the observed v alues, while a significant discrepancy is found at the Ca2+ sites. Mul liken analysis indicates that the spin density at the proton is mainly due to the core polarization of the H2- is orbital.