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We investigate from first principles the proposed destruction of the controversial e(')g pockets in the Fermi surface of Na(x)CoO(2) due to Na disorder, by calculating its k-dependent configuration-averaged spectral function . To this end, a Wannier function-based method is developed that treats the effects of disorder beyond the mean field. Remarkable spectral broadenings of order ∼1  eV are found for the oxygen orbitals, possibly explaining their absence in the experiments. In contradiction with the current claim, however, the e(')g pockets remain almost perfectly coherent. The developed method is expected to also generate exciting opportunities in the study of the countless functional materials that owe their important electronic properties to disordered dopants.

The relative importance of electron-lattice (e-l) and electron-electron (e-e) interactions in ordering orbitals in LaMnO3 is systematically examined within the local-density approximation + Hubbard U approximation of density functional theory. A realistic effective Hamiltonian is derived from novel Wannier state analysis of the electronic structure. Surprisingly, e-l interaction (approximately or = 0.9 eV) alone is found insufficient to stabilize the orbital ordered state. On the other hand, e-e interaction (approximately or = 1.7 eV) not only induces orbital ordering, but also greatly facilitates the Jahn-Teller distortion via enhanced localization. Further experimental means to quantify the competition between these two mechanisms are proposed.