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Assessing density functionals using many body theory for hybrid perovskites

Published in Physical Review Letters

One of the most important issues in the modelling of materials is the choice of an appropriate density functional. Many researchers employ functionals commonly used in their field or they have some sort of chemical intuition, why one density functional should be preferred over another one. This is an ill-advised strategy and a better approach is necessary. In this paper, we present a concise approach to select the best functional for a particular materials system using a state of the art method beyond density functional theory (DFT). We use the random phase approximation (RPA), which is placed one step above hybrid functionals on the metaphorical 'Jacob's Ladder' towards the exact total energy. This method has an accuracy unattained by any other computational method for solids. A breakthrough in the analytical formulation of forces in the RPA earlier this year makes this novel approach possible. An efficient implementation in the VASP code allows to perform molecular dynamics at the RPA level, something that seemed impossible just a few years ago. A finite temperature ensemble of realistic crystal structures and the associated energies are calculated. Comparing these energies to the ones obtained with commonly used density functionals allows to rank them based on their accuracy.

To verify this new approach, we study one of the most exciting novel light-harvesting materials: MAPbI3. Its structure is a particularly hard nut to crack for DFT. This is due to the large dynamical degree of freedom of the Methyl-ammonium molecules and the interplay of van der Waals forces and cage instabilities in the perovskite structure. A surprising outcome is that the SCAN functional outperforms a computationally much more costly hybrid functional. Furthermore, with SCAN we are for the first time able to show the nature of the dynamically stable tetragonal phase of MAPbI3 observed at room temperature.

Menno Bokdam, Jonathan Lahnsteiner, Benjamin Ramberger, Tobias Schäfer and Georg Kresse, Phys. Rev. Lett. 119, 145501 – Published 6 October 2017
Journal: https://doi.org/10.1103/PhysRevLett.119.145501
Open access: https://arxiv.org/abs/1708.06821

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