Molecular open shell configuration interaction calculations using the Dirac–Coulomb Hamiltonian: The f ⁶‐manifold of an embedded EuO⁹⁻₆ cluster

We present results of all‐electron molecular relativistic (Hartree–Fock–Dirac) and nonrelativistic (Hartree–Fock) calculations followed by a complete open shell configuration interaction (COSCI) calculation on an EuO⁹⁻₆ cluster in a Ba₂GdNbO₆ crystal. The results include the calculated energies of a number of states derived from the f⁶−manifold and ⁵D–⁷F luminescence transition wavelengths. The calculations were performed using the molecular Fock–Dirac (molfdir) program package developed in our laboratory. The theory and methods employed in this package are briefly described. The physical models used to analyze the Eu³⁺ impurity states range from a bare Eu³⁺ ion to an EuO⁹⁻₆ cluster embedded in a Madelung potential representing the rest of the crystal. We show that it is necessary to use the embedded cluster model to get a reasonable description of the crystal field splittings of the states arising from the f⁶‐manifold. Our results indicate that the calculated splittings are very sensitive to the orbitals used. It is therefore essential that relativistic orbitals be used from the outset.