Introducing the ELF topological analysis in the field of quasirelativistic quantum calculations

We present an original formulation of the electron localization function (ELF) in the field of relativistic two-component DFT calculations. Using I2 and At2 species as a test set, we show that the ELF analysis is suitable to evaluate the spin-orbit effects on the electronic structure. Beyond these examples, this approach opens up new opportunities for the bonding analysis of large molecular systems involving heavy and super-heavy elements

Design Of Next Generation Force Fields From AB Initio Computations: Beyond Point Charges Electrostatics

International audienc

Design Of Next Generation Force Fields From AB Initio Computations: Beyond Point Charges Electrostatics

International audienc

Possible survival of simple amino acids to X-ray irradiation in ice: the case of glycine

International audienceContext. Glycine, the simplest of amino acids, has been found in several carbonaceous meteorites collected on Earth, though its presence in the interstellar medium (ISM) has never been confirmed as of today. It is now considered that its synthesis took place in the icy mantles of interstellar grains, but it remains unclear how glycine, once synthesized and trapped in interplanetary particles, survives during the transfer to the Earth.Aims. Assuming that glycine was effectively formed in the ice, we address the question of its resistance to a solar-like radiation field and look for the possible molecular remnants that would be useful tracers of its former existence.Methods. The search was conducted using an interdisciplinary approach that mixes, on the one hand, irradiations in ultra high vacuum at 30 K on the TEMPO beam line of the synchrotron SOLEIL, simultaneously with near-edge X-ray absorption spectroscopy (NEXAFS) measurements, and on the other hand, quantum calculations to determine the energetics of the fragmentations and the relative stability of the different byproducts. The last points were addressed by means of density functional theory (DFT) simulations followed by high-level post Hartree-Fock calculations when more accurate relative energies were necessary. The constraints of an icy environment deserved special attention and the ice was modeled by a polarizable continuum medium that relies on the dielectric constant of water ice at 10–50 K.Results. Destruction of glycine is observed in the first seconds of irradiation, and carbon dioxide (CO2) and methylamine (CH3NH2) are formed. Carbon monoxide (CO), methanimine (CH2NH) and hydrogen cyanide (HCN) are also produced in secondary reactions. The amino acid destruction is the same for pure glycine and glycine in ice, indicating that the OH radicals released by the water matrix is barely involved in the photolytic process; however, these radicals are involved in the production of the secondary byproducts through dehydrogenation reactions as shown by ab initio quantum chemical simulation presented in this article along with the experimental results.Conclusions. The experiments show that glycine is only partially destroyed. Its abundance is found to stay at a level of ~30% of the initial concentration, for an irradiation dose equivalent to three years of solar radiation (at a distance of one astronomical unit). This result supports the hypothesis that, if trapped in protected icy environments and/or in the interior of interplanetary particles and meteorites, glycine may partly resist the radiation field to which it is submitted and, accordingly, survives its journey to the Earth

Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association

International audienceThe deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H+3H3+ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H+3H3+, especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H+3H3+ only, considering that the protonated ions produced in the destruction of H+3H3+ by its main competitors present in the primitive nebula, i.e., H2O, CO, and N2, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H2O, CO, and N2, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10−16 to 10−19 cm3 s−1, which is reasonable for such reactions and has to be compared to the rates of 10−16 to 10−18 cm3 s−1, obtained with H+3H3+. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe

Nine questions on energy decomposition analysis

The paper collects the answers of the authors to the following questions: 1. Is the lack of precision in the definition of many chemical concepts one of the reasons for the coexistence of many partition schemes? 2. Does the adoption of a given partition scheme imply a set of more precise definitions of the under-lying chemical concepts? 3. How can one use the results of a partition scheme to improve the clarity of definitions of concepts? 4. Are partition schemes subject to scientific Darwinism? If so, what is the influence of a community's sociological pressure in the "natural selection" process? 5. To what extent does/can/should investigated systems influence the choice of a particular partition scheme? 6. Do we need more focused chemical validation of EDA methodology and descriptors/terms in general? 7. Is there any interest in developing common benchmarks and test sets for cross-validation of methods? 8. Is it possible to contemplate a unified partition scheme (let's call it the "standard model" of partitioning) that is proper for all applications in chemistry, in the foreseeable future or even in principle? 9. In the end, science is about experiments and the real world. Can one therefore use any experiment or experimental data be used to favor one partition scheme over another