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.