  Ytterbium-based delafossites with effective S=1/2 moments are investigated
intensively as candidates for quantum spin-liquid ground states. While the
synthesis of related cerium compounds has also been reported,many important
details concerning their crystal, electronic, and magnetic structures are
unclear. Here we analyze the S=1/2 system KCeO2, combining complementary
theoretical methods. The lattice geometry was optimized and the band structure
investigated using density functional theory extended to the level of a GGA+U
calculation in order to reproduce the correct insulating behavior. The Ce 4f1
states were then analyzed in more detail with the help of ab initio
wave-function-based computations. Unusually large effective crystal-field
splittings of up to 320 meV are predicted, which puts KCeO2 in the strong field
coupling regime. Our results reveal a subtle interplay between ligand-cage
electrostatics and the trigonal field generated by the extended crystalline
surroundings, relevant in the context of recent studies on tuning the nature of
the ground-state wave function in 4f triangular-lattice and pyrochlore
compounds. It also makes KCeO2 an interesting model system in relation to the
effect of large crystal-field splittings on the anisotropy of intersite
exchange in spin-orbit coupled quantum magnets.
