"GW and Bethe-Salpeter equation calculations with FIESTA code"
by Ivan Duchemin, CEA-INAC
Organic photovoltaic systems present the specificity that the photo-generated excitons are strongly bound, justifying the realisation of donor/acceptor interfaces at which the charge separation can be achieved. The exact processes associated with such a process is however still a matter of discussion. As such, besides the standard goal of describing correctly the electronic and optical properties of organic semiconductors, one needs to describe correctly the band offsets at the interface and understand the related electron-hole dissociation processes. As will be shown in this presentation, the standard ab initio tools, namely DFT and its time-dependent extension (TDDFT) present severe limitations. We will show that the so-called GW and Bethe-Salpeter formalisms, allow to cure much of the problems associated with the (TD)DFT formalisms. Our GW and Bethe-Salpeter implementation with Gaussian bases allows to tackle systems comparison a few hundred atoms. The numerical implementation details and its scaling will be presented. Finally, perspective such as the accouting of the environment effect on electronic properties will be discussed.
 "First principles calculations of charge transfer excitations in polymer-fullerene complexes: Influence of excess energy", D. Niedzialek, I. Duchemin, T. Branquinho de Queiroz, S. Osella, A. Rao, R. Friend, X. Blase, S. Kümmel, and D. Beljonne, Adv. Funct. Mater. 25 pp. 1287-1295 (2015).
 "Many-body Green's function study of coumarins for dye-sensitized solar cells", C. Faber, I. Duchemin, T. Deutsch, X. Blase, Phys. Rev. B, 86, 155315 (2012)
 "Short-range to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: A Bethe-Salpeter study", I. Duchemin, T. Deutsch, X. Blase, Phys. Rev. Lett. 109, 167801 (2012).
 "First-principles GW calculations for DNA and RNA nucleobases", Carina Faber, Claudio Attaccalite, V. Olevano, E. Runge, X. Blase, Phys. Rev. B 83, 115123 (2011).
"Solvation at the molecular scale, the MDFT route",
by Maximilien Levesque (ENS-Paris et Maison de la Simulation)
Processes taking place in the liquid state, for instance chemical reactions, happen in a sea of solvent molecules. They are legion, and some say they don't forget, but can we predict their effect in solution?
(i) Roughly yes, using rough methods that rely on a macroscopic description of the solvent. They are fast (say few cpu-seconds) but are not able to capture the physical, molecular nature of solvation. No packing, no orientation effects, no hydrogen-bonding, among others.
(ii) Yes, accurately, using explicit simulations like molecular dynamics. But these are at least 3 to 4 orders of magnitude slower. Hundreds or thousands of cpu-hours are often necessary.
(iii) We will present the molecular density functional theory and its associated code, MDFT. We will show how state-of-the-art liquid state theory and high performance algorithms can capture solvation effects at their inherent molecular scale, for the cost of rough methods.