Interaction of Metamitron and Fenhexamid with Ca2+‐ Montmorillonite Clay Surfaces: A Density Functional Theory Molecular Dynamics Study

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Desorption Mechanism of a Pesticide from a Hydrated Calcium Montmorillonite Unraveled by Molecular Dynamics Simulations

Fenhexamid, N-(2,3-dichloro-4-hydroxyphenyl)-1-methylcyclohexanecarboxamide, a fungicide, is authorized in the European Union to be used in agriculture. Active molecule behavior in the environment and its transfer to environmental components (soil, water, and air) are primordial topics to study to limit environmental pollution. Following a step by step approach, we studied fenhexamid. In previous works, we worked on the fenhexamid pesticide isolated and then on its complexes with one or two Ca2+ and Na+, the most abundant cations in soils, in the gas phase. We also reported theoretical calculations about fenhexamid in interaction with a clay surface: Ca–montmorillonite. In the present paper, potentials of mean force describing overall fenhexamid desorption from a Ca–montmorillonite surface in the presence of water were computed from molecular dynamics simulations combined with umbrella sampling. Several mechanisms were envisaged. The results allow to quantitatively estimate the free energy of desorption and to provide a detailed insight into the hydration sphere of Ca2+ during the desorption proces

Direct selected multireference configuration interaction calculations for large systems using localized orbitals

International audienceA selected multireference configuration interaction (CI) method and the corresponding code are presented. It is based on a procedure of localization that permits to obtain well localized occupied and virtual orbitals. Due to the local character of the electron correlation, using local orbitals allows one to neglect long range interactions. In a first step, three topological matrices are constructed, which determine whether two orbitals must be considered as interacting or not. Two of them concern the truncation of the determinant basis, one for occupied/virtual, the second one for dispersive interactions. The third one concerns the truncation of the list of two electron integrals. This approach permits a fine analysis of each kind of approximation and induces a huge reduction of the CI size and of the computational time. The procedure is tested on linear polyene aldehyde chains, dissociation potential energy curve, and reaction energy of a pesticide-Ca2+ complex and finally on transition energies of a large iron system presenting a light-induced excited spin-state trapping effect

Theoretical study of the atrazine pesticide interaction with pyrophyllite and Ca2+-montmorillonite clay surfaces

International audienceAtrazine, a pesticide belonging to the s‐triazine family, is one of the most employed pesticides. Due to its negative impact on the environment, it has been forbidden within the European Union since 2004 but remains abundant in soils. For these reasons, its behavior in soils and water at the atomic scale is of great interest. In this article, we have investigated, using DFT, the adsorption of atrazine onto two different clay surfaces: a pyrophyllite clay and an Mg‐substituted clay named montmorillonite, with Ca2+ compensating cations on its surface. The calculations show that the atrazine molecule is physisorbed on the pyrophyllite surface, evidencing the necessity to use dispersion‐corrected computational methods. The adsorption energies of atrazine on montmorillonite are two to three times larger than on pyrophyllite, depending on the adsorption pattern. The computed adsorption energy is of about −30 kcal mol−1 for the two most stable montmorillonite‐atrazine studied isomers. For these complexes, the large adsorption energy is related to the strong interaction between the chlorine atom of the atrazine molecule and one of the Ca2+ compensating cations of the clay surface. The structural modifications induced by the adsorption are localized: for the surface, close to substitutions and particularly below the Ca2+ cations; in the molecule, around the chlorine atom when Ca2+ interacts strongly with this basic site in a monodentate mode. This study shows the important role of the alkaline earth cations on the adsorption of atrazine on clays, suggesting that the atrazine pesticide retention will be significant in Ca2+‐montmorillonite clays

Low-lying excited states of model proteins: Performances of the CC2 method versus multireference methods

International audienceA benchmark set of relevant geometries of a model protein, the N-acetylphenylalanylamide, is presented to assess the validity of the approximate second-order coupled cluster (CC2) method in studying low-lying excited states of such bio-relevant systems. The studies comprise investigations of basis-set dependence as well as comparison with two multireference methods, the multistate complete active space 2nd order perturbation theory (MS-CASPT2) and the multireference difference dedicated configuration interaction (DDCI) methods. First of all, the applicability and the accuracy of the quasi-linear multireference difference dedicated configuration interaction method have been demonstrated on bio-relevant systems by comparison with the results obtained by the standard MS-CASPT2. Second, both the nature and excitation energy of the first low-lying excited state obtained at the CC2 level are very close to the Davidson corrected CAS+DDCI ones, the mean absolute deviation on the excitation energy being equal to 0.1 eV with a maximum of less than 0.2 eV. Finally, for the following low-lying excited states, if the nature is always well reproduced at the CC2 level, the differences on excitation energies become more important and can depend on the geometry