Maximum volume simplex method for automatic selection and classification of atomic environments and environment descriptor compression.

Fingerprint distances, which measure the similarity of atomic environments, are commonly calculated from atomic environment fingerprint vectors. In this work, we present the simplex method that can perform the inverse operation, i.e., calculating fingerprint vectors from fingerprint distances. The fingerprint vectors found in this way point to the corners of a simplex. For a large dataset of fingerprints, we can find a particular largest simplex, whose dimension gives the effective dimension of the fingerprint vector space. We show that the corners of this simplex correspond to landmark environments that can be used in a fully automatic way to analyze structures. In this way, we can, for instance, detect atoms in grain boundaries or on edges of carbon flakes without any human input about the expected environment. By projecting fingerprints on the largest simplex, we can also obtain fingerprint vectors that are considerably shorter than the original ones but whose information content is not significantly reduced.

Playing with isomerism and N substitution in pentalenedione derivatives for organic electrode batteries: how high are the stakes?

New concepts to design innovating and top-performing redox-active organic molecules based electrodes should push forward and promote an eco-friendly alternative to classical Li-ion batteries. In this promising research area, density functional theory calculations lend support to experiments through the prediction of redox voltage and give promise to rationalize the trends, thus providing a general approach for engineering advanced materials. In this study in which we analysed spin density/net atomic charges distribution along with global energy decomposition thanks to Bader's partitioning of the molecular space, a vision for designing pentalenedione derivatives by fine tuning of the redox potential properties is presented. The concept relies on combined effects of isomerism and N single/double substitution for CH on the parent backbone. Such dual nature modification is able to provide a series of compounds within the range of 2.2-3.6 V vs. Li(+)/Li (against a more restricted range of 2.2-2.8 V vs. Li(+)/Li for the sole effect of isomerism on the unsubstituted parent compounds). The incidence of double N substitution alone generally follows an almost additive rule based on the combined actions of the composing single N substitutions. Few exceptions to the rule were, however, also observed and rationalized. Beyond learning gained for this peculiar family, these results may have exciting implications for future design strategies.

A study into the effect of subtle structural details and disorder on the terahertz spectrum of crystalline benzoic acid.

The phonon modes of crystalline benzoic acid have been investigated using terahertz time-domain spectroscopy, rigid molecule atom-atom model potential and plane-wave density functional theory lattice dynamics calculations. The simulation results show good agreement with the measured terahertz spectra and an assignment of the terahertz absorption features of benzoic acid is made with the help of both computational methods. Focussing on the strongest interactions in the crystal, we describe each vibration in terms of distortions of the benzoic acid hydrogen bonded dimers that are present in the crystal structure. The terahertz spectrum is also shown to be highly sensitive to the location of the carboxylic acid hydrogen atoms in the cyclic hydrogen-bonded dimers and we have systematically explored the influence of the observed disorder in the hydrogen atom positions on the lattice dynamics.