Ab initio DFT: Getting the right answer for the right reason

Some highlights of the WATOC plenary lecture are presented. We discuss what ab initio dft is, and that unlike standard density functional methods (DFT) methods, it has to converge to the right answer in the limit of basis and correlation like ab initio wavefunction methods. We obtain the correct behavior of the exchange and correlation potentials. In the case of correlation in particular, these are vastly different than those obtained by standard methods like LYP and PBE, and sufficiently different for exchange, that the numerical effect of the correct potentials is quite important. With the correct potentials, the orbital energies in Kohn-Sham DFT are shown to approximately satisfy a kind of Koopmans' theorem, while at the same time, the differences between the occupied orbital energies and the unoccupied orbital energies, offers an excellent zeroth-order approximation to the excitation energy. We demonstrate that we can build a local correlation potential using an optimized effective potential (OEP) strategy, that is equally applicable to non-local Hartree-Fock exchange as it is to local exchange. This makes it possible to create consistent 'hybrid' dft methods at the level of the potential. Some numerical consequences are discussed.