The study of photoexcitations in molecular aggregates faces the twofold problem of the increased computational cost associated with excited states and the complexity of the interactions among the constituent monomers. A mechanistic investigation of these processes requires the analysis of the intermolecular interactions, the effect of the environment, and 3D arrangements or crystal packing on the excited states. A considerable number of techniques have been tailored to navigate these obstacles; however, they are usually restricted to in‐house codes and thus require a disproportionate effort to adopt by researchers approaching the field. Herein, we present the FRamewOrk for Molecular AGgregate Excitations (fromage), which implements a collection of such techniques in a Python library complemented with ready‐to‐use scripts. The program structure is presented and the principal features available to the user are described: geometrical analysis, exciton characterization, and a variety of ONIOM schemes. Each is illustrated by examples of diverse organic molecules in condensed phase settings. The program is available at https://github.com/Crespo-Otero-group/fromage.
A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the 'hard sphere' approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A site cavity of the correct size, and maintaining suitable anion-anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighbouring octahedra show very significant anion-anion overlap. We use these models to predict a compound with A site rattling and use DFT to confirm this prediction. Lastly, we use the geometric models to derive formability criteria for vacancy ordered double perovskites.
Using ab initio methods, we predict α-Bi2Sn2O7 to have an ultra-low lattice thermal conductivity at room temperature due to the high density of phonon scattering events, which makes it a potential earth-abundant n-type low temperature thermoelectric.
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