A reprint of the Poster (PDF) is available here.
A single Metyl-Ammonium at the centre of a 2x2x2 supercell expansion is represented.
2x2x2 Supercell; based on an expansion of a relaxed Pseudo-cubic
PBESol functional (corrected GGA)
Gamma point calculation
0.5 fs integration timestep for molecular dynamics
58 ps / 2319 frames total simulation time (this analysis)
~120 hours of computation on 96 cores on EPSRC ARCHER
Custom C codes evaluating classical Hamiltonian on a cubic lattice. Due to the simplicity of the Hamiltonian, many million moves can be attempted per second. This allows for a study of relatively long time frames, with fairly equilibrated statistics, on device-sized segments of the Perovskite material.
The code is now fully 3D; data presented in the paper is 2D as this is easier to understand visually.
So far the parameters are calculatd for the methyl-ammonium ions rotating themselves, but equivalent physics would be produced by the inorganic cage distorting and losing centrosymmetry itself.
By introducing an additional, local, strain term into our Hamiltonian (parameterised from a 4x4x4 supecell periodic DFT calculation of aligned MA domains, where we flip a single MA ion), we can induce the formation of fully ferroelectric domains.
The degree of overall alignment of the simulation volume can be easily tracked by defining a Landau order parameter, the most simple of which is simply the vector average of the MA alignments. For a fully aligned domain, the length of this vector is 1, otherwise ~0.
The above video shows the MA domains on the left, with the resulting dipole electrostatic potential on the right. The time (or more formally, the number of Monte-Carlo moves attempted per frame) accelerates geometrically, as the ripening process of the domains becomes progressively slower.
MAPI MD video download (Figshare)
Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells  APL Mat. 2, 081506 (2014); http://dx.doi.org/10.1063/1.4890246
Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells Nano Lett., 2014, 14 (5), pp 2584–2590 http://dx.doi.org/10.1021/nl500390f