Lessons learned from urgent computing in Europe: Tackling the COVID-19 pandemic.

PRACE (Partnership for Advanced Computing in Europe), an international not-for-profit association that brings together the five largest European supercomputing centers and involves 26 European countries, has allocated more than half a billion core hours to computer simulations to fight the COVID-19 pandemic. Alongside experiments, these simulations are a pillar of research to assess the risks of different scenarios and investigate mitigation strategies. While the world deals with the subsequent waves of the pandemic, we present a reflection on the use of urgent supercomputing for global societal challenges and crisis management.

Through versus cross electron delocalization in polytriacetylene oligomers: a computational analysis.

Herein, we report a systematic theoretical investigation of the molecular and electronic properties of unsubstituted polytriacetylene (PTA) and iso-polytriacetylene (iso-PTA) oligomers, which are characterized by through and cross pi-conjugation pathways, respectively. The goal of the study is to compare through versus cross conjugation on the basis of the computed molecular geometries of the neutral, anionic, and cationic species, the electron affinities, ionization potentials, excitation energies, and nonlinear optical properties for oligomers up to the nonamer. Differences in the effective conjugation length are directly related to electron delocalization in cross- and through-conjugated pathways. As in the through-conjugated oligomers, that is, the PTAs, the frontier orbitals of the iso-PTA oligomers are delocalized along the entire carbon backbone, suggesting that pi-delocalization can extend through cross-linked carbon atoms. However, in contrast to the PTA oligomers, the bond lengths remain strictly constant and the reduction of the energy gap beyond the trimer is completely due to the correlation contribution. On the other hand, in the anions and cations, the bond lengths do change significantly with increasing chain length. Therefore, oxidation or reduction of the iso-PTA oligomer appears to switch on delocalization through cross-linked carbon atoms. Obviously, the effective conjugation length is specific and depends on the observable considered.

Electron delocalization in linearly pi-conjugated systems: a concept for quantitative analysis.

Donor- and/or acceptor-substituted pi-conjugated systems represent an important class of compounds in organic chemistry. However, up to now, a general method to quantitatively address the efficiency of a conjugated path is still missing. In this work, a novel computational approach based on deletion energies and on second-order orbital interaction energies in a natural bond orbital (NBO) scheme is employed to quantitatively assess ("measure") delocalization energies. Moreover, the purpose of this work is to assess the efficiency of distinct pi-conjugated paths, that is, geminal, cis, and trans, as well as to predict the impact of substituents on a given backbone. This study is focused on various mono-, di-, tri-, and tetrasubstituted tetraethynylethenes (TEEs). These model systems are suitable for our analysis, because they offer distinct conjugation paths within the same molecule, and can also be substituted in multiple ways. Differences between conjugation paths, the effect of neighbor paths, and the impact of donor and acceptor substituents on the various paths are discussed.

trans versus geminal electron delocalization in tetra- and diethynylethenes: a new method of analysis.

trans-Diethynylethene [(E)-hex-3-ene-1,5-diyne (1 a)], geminal-diethynylethene [3-ethynyl-but-3-ene-1-yne (1 b)], and tetraethynylethene [3,4-diethynyl-hex-3-ene-1,5-diyne (2)] are flexible molecular building blocks for pi-conjugated polymers with interesting electronic and photonic properties. The type of functionalization, the length of the polymer chain, and the choice of pi-conjugation pattern, play a crucial role in determining the properties of these compounds. To rationalize the impact of the different delocalization pathways in the various types of isomers (trans or geminal) on the molecular and electronic structure, a detailed theoretical investigation is presented. We develop a method based on the natural bond orbital (NBO) analysis of Weinhold, which allows one to correlate electron delocalization with molecular and electronic structure observables. The method reveals that the difference between trans (or through) and geminal (or cross) conjugation is not only due to the vertical pi conjugation, but also to the in-plane sigma hyperconjugation. The method is used to correlate the changes in molecular and electronic observables, such as the bond lengths or the absorption frequencies, with the electronic structure of the compounds under investigation. Moreover, this method allows us to predict how a certain substituent will affect the molecular structure and the electronic properties of a given backbone.