Confirmed Participants: Thomas Ayral, Arjan Berger, Silke Biermann, Christian Brouder, Michele Casula, Pierluigi Cudazzo, Ismaila Dabo, Kay Dewhurst, Stefano Di Sabatino, Christoph Friedrich, Matteo Gatti, Matteo Guzzo, Philipp Hansmann, Josh Kas, Giovanna Lani, Cyril Martins, Valerio Olevano, Lucia Reining, Patrick Rinke, Pina Romaniello, Claudia Rödl, Priyanka Seth, Sangeeta Sharma, Francesco Sottile, Lorenzo Sponza, Adrian Stan, Falk Tandetzky, Walter Tarantino
DISCUSSION
WEDNESDAY 05-12-2012
Arjan Berger: Solution of the nonlinear problem in one point
In Ref.[1] Lani et al. explore a linearized version of a set of coupled nonlinear first order differential equations which related the one-body Green's function to it's functional derivative with respect to an external potential (which is eventually taken to zero). The linearized set of differential equations is studied in the so-called one-point model, where an analytical solution is possible, which provides valuable insights on the solution of the full functional problem. In this work we study the original nonlinear set of equations in the 1-point model, following the same line of Ref. [1] for the linearized case. [1] G. Lani, P. Romaniello, and L. Reining New J. Phys. 14, 013056 (2012)
Patrick Rinke: SOSEX self-energy
Despite the success of the GW method in describing the photoemission spectra of solids, molecules and clusters, challenges remain. For aromatic molecules for example absolute as well as relative positions of ionisation energies and affinities are not well reproduced in perturbative G0W0 schemes with different starting points as well as in self-consistent GW [1], sometimes even giving the wrong orbital order. Motivated by renormalized second-order perturbation theory [2] for the ground-state energy, we propose a second-order screened exchange correction (SOSEX) to the GW self-energy. This correction follows the spirit of the SOSEX correction to the random-phase approximation for the electron correlation energy and reduces the self-correlation error. The performance of the GW+SOSEX scheme has been benchmarked for a set of molecular systems, including the G2 set, commonly used acceptor molecules, benzene and the azabenzene molecules. We find that the SOSEX correction improves the description of the spectral properties including the orbital order with respect to the different GW schemes, highlighting the importance of reducing the self-correlation error. [1] N. Marom et al., arXiv:1211.0416 [2] X. Ren, P. Rinke, C. Joas, and M. Scheffler, J. Mat. Sci. 47, 7447 (2012)
Ismaila Dabo: Limitations of density-functional theory approximations in describing molecular dissociations and electron transfer
Electronic-‐structure calculations based upon density-‐functional theory (DFT) have been fruitful in diverse areas of condensed matter physics. Despite their exceptional success, it can hardly be denied that a range of fundamental electronic properties fall beyond the scope of current DFT approximations. (1) Many of the failures of DFT calculations take root in the lack of piecewise linearity of approximate functionals, (2) which reverberates negatively on the electronic-‐structure description of systems involving fractionally occupied and spatially delocalized electronic states, including but not restricted to dissociated molecules, adsorbed species, charge-‐transfer complexes, and semiconducting compounds. In this talk, I will present a novel class of first-‐principles methods that restores the piecewise linearity of the total energy by imposing Koopmans’ theorem to DFT approximations. (3) The Koopmans-‐compliant approach is apt at describing full orbital spectra within a few tenths of an electron-‐volt relative to experimental direct and inverse photoemission data. This level of accuracy is comparable to the predictive performance of accurate many-‐body perturbation theory methods at a fraction of their computational cost — and with the additional benefit of providing accurate total energies for systems with fractional occupations. Time permitting, I will discuss several applications, including the description of charge transfer in organic heterojunctions and phosphorescent heavy-‐metal dyes in light of the restored piecewise linearity.(4,5) (1) Cohen A. J., Mori-‐Sanchez P., Yang W., Insights into current limitations of density-‐functional theory, Science 321, 792 (2008) (2) Perdew J. P., Parr R. G., Levy M., Balduz J. L., Density-‐functional theory for fractional Particle number: derivative discontinuities of the energy, Physical Review Letters 49, 1691 (1982). (3) Dabo I., Ferretti A., Poilvert N., Li Y. L., Marzari N., Cococcioni M., Koopmans’ condition for density-‐functional theory, Physical Review B 82, 115121 (2010) (4) Dabo I., Ferretti A., Park C.-‐H., Poilvert N., Cococcioni M., Marzari N., Donor and acceptor levels of organic photovoltaic compounds from first principles, Physical Chemistry and Chemical Physics 15, 685 (2013) (5) Himmetoglu B., Marchenko A., Dabo I., Cococcioni M., Role of electronic localization in the phosphorescence of iridium sensitizing dyes, Journal of Chemical Physics 137, 154309 (2012)
Walter Tarantino: Time-dependent density functional theory for quantum electrodynamics
The project is aimed to develop a time-dependent functional theory for quantum electrodynamics. We want to establish a Kohn -Sham system for relativistic electrons, as well as positrons and photons. Apart from representing the natural generalization of the usual non-relativistic many-body problem, the approach will provide us with a novel instrument for computational investigations of the interaction between light and matter. Details about preliminary results on KS systems and the effective (exchange-correlation) potentials will be presented.
Christoph Friedrich: Incomplete-basis-set correction for response functions
We present an incomplete-basis-set correction (IBC) for all-electron response functions within the full-potential linearized augmented plane-wave (FLAPW) method as realized in the FLEUR code and apply this correction to the exact-exchange (EXX) optimized-effective-potential (OEP) approach. The IBC utilizes the potential dependence of the LAPW basis functions whose response is calculated explicitly by solving radial Sternheimer equations in the spheres, allowing to go beyond the Hilbert space spanned by the basis set. We demonstrate that this IBC improves the convergence in terms of basis-set size and number of unoccupied states. As an illustration, we show calculations of the transition-metal oxides MnO, FeO, CoO, and NiO, which are correctly predicted to be large-gap insulators.
20:00 Dinner at the restaurant Le Gramophone in Orsay (http://www.legramo.com/)
THURSDAY 06-12-2012:
Joshua Kas: Cumulant Expansion For the Electron Green's Function: Phonon Excitations
Here we present work on phonon contributions to the electron self-energy and the effect on the electron Green's function. In particular we compare the GW form of the spectral function to that of the cumulant expansion as a function of quasiparticle energy and temperature.
Philipp Hansmann and Thomas Ayral: Ab-initio calculations for ad-atoms on surfaces : insights from cRPA and GW+DMFT