"I'm after the cleanest line on the steepest part of the face" - Y. Chouinard
The research activity of the Modélisation des Systèmes Complexes team (MSC) focuses on two main areas: research and validation of methodologies based on DFT, and DFT method validation on large and real systems of electrochemical and/or chemical interest.
The first main thematic of research concerns the development and the implementation of new functionals for exchange and correlation. The second is to study the effect of self-interaction error (SIE) on some exchange functionals, and to correct this effect. A final and recent area of development concerns the implementation of a spin-orbit coupling correction in the Gaussian program package.
The second main thematic of research focuses on the DFT method applications developed to analyze the properties of complex chemical systems. In particular, we have focused our efforts on two main issues, the dye sensitized cells (in collaboration with the team of Dr. Lincot) and the electrochemical sensors based on metal macro-complexes (in collaboration with the team of Dr. F. Bedioui).
Modelling Proton Transfer in Polymer Electrolyte Membrane Fuel Cell (PEMFC)
The performance of PEMFCs mainly depends on the proton conductivity of the polymer membrane used as electrolyte. An improvement of the knowledge of mechanisms involved in proton transfer can be very useful in order to develop new materials. Where the experimental knowledge is until insufficient, molecular modeling represent avery interesting way to study proton conductivity.
Dye Sensitized Solar Cells
Our research mainly focus on the ab-initio modeling (essentially based on Density Functional Theory, DFT) of dye-sensitized solar cells, whose operating principles rely on electronic process localized at the dye semi- conductor interface, where an ultrafast electron injection takes place between the dye excited state and the semiconductor conduction band. An efficient computational protocol has been developed to quantitatively describe both isolated and adsorbed systems. To this end, UV-Visible spectra of dyes are simulated using time-dependent DFT and a polarizable continuum solvation model, while both the isolated semiconductor and combined dye/semiconductor systems are investigated using large supercells under periodic boundary conditios. Electron injection times are computed using a simple orbital-based model. Large clusters are also extracted from obtained periodic structures in order to study the semiconductor influence on the dyes' UV-Visible spectra.
Molecular Modelling and Kinetic of the Peroxydation Process of Organic Compounds
A great number of organic compounds spontaneously decompose, by a free-radical reaction of the carbon chain with molecular oxygen, in a self-propagating process of auto-oxidation, which may generate a wide variety of peroxide molecules. Many laboratory accidents have been ascribed to the presence of peroxides in solvents or reagents. Among the chemicals widely used in a lot of laboratories and industries as solvent or reactive, ethers are the most notorious peroxide formers causing several accidents. However, few of works propose mechanistic study of their reaction of oxidation.
Solid Oxide Fuel Cells
Solid Oxide Fuel Cells (SOFCs) are a promising technology for direct or indirect conversion of a wide range of hydrocarbons into electrical energy, enabling increased efficiency over traditional power generation systems. Moreover, SOFCs cause reduced pollution such as NOx and SOx upon operation. One of the main limitation is however the need of efficient catalysts at the anode for hydrocarbons oxidation that also match all conductivity and compatibility with other materials requirements. The high working temperature of SOFCs (800-1000K) enables the use of cheaper catalysts such as cerium oxide to oxidize the fuel, avoiding the use of expensive noble metals. Cerium oxide (CeO2) crystallizes in the face-centred cubic form (Fm3m). Oxygen vacancies (VO) can be easily formed and healed, leading to the well-know "oxygen storage capacity" of ceria (OSC). Each oxygen vacancy leaves behind a pair of electrons that have been shown to localize into atomic-like Ce4f orbitals of two neighbouring Cerium. Electronic structure of ceria shows two band gaps: (i) from O 2p states (valence band) to localized, atomic-like Ce 4f states and (ii) from O 2p to Ce 5d states (conduction band). Qualitative correct description of the electronic structure of both oxidation states of ceria with a single DFT functional is challenging. Pure DFT calculations tend to over delocalize these electrons, predicting an incorrect conducting state for reduced ceria. Hybrid approaches such as PBE0 manage to predict the correct insulating states for both stoichiometric and reduced ceria and gives quantitatively reasonable values for both band gaps. Moreover, PBE0 can also be used to describe molecular species such as hydrogen, methane, enabling the study of catalytic properties of ceria for fuels oxidation.
Prediction of UV-visible Spectra
In pharmaceutical chemistry, UV-visible absorption spectra are routinely used in the quantitative determination of organic compounds in solution. When a UV-visible spectrometer is employed as a detector for HPLC-UV analysis, the presence of analyte gives a response which is assumed to be proportional to the concentration, using chemical standards to quantify the substance. However, the quantitative analysis of impurities requires its isolation or its synthesis, a time demanding and costly procedure. The simulation of UV-visible spectra by computational chemistry tools is particularly appealing since modern approaches are able to provide results with an accuracy comparable to that obtained by experiments (about 0.1 eV). In this sense, methods based on Time-Dependent Density Functional Theory (TD-DFT) provide very accurate results. Such approaches (as well as other quantum chemical methods) allow for the calculation of electronic transitions between the ground state and the different excited states which give the energies (hence the wavelength) of the corresponding radiations. The intensities are then evaluated from the oscillator strengths, these latter being obtained from the transition moments. However, quantum mechanics provides UV-visible spectrum as stick spectrum, but due to different factors such as natural line width, Doppler effect, pressure, vibronic effect and also thermal excitement, each transition can be enlarged with a gaussian shape. In this context an integrated approach has been developed for the simulation of UV-visible absorption spectra. Starting from the stick transitions obtained by TD-DFT calculation, the spectra are then obtained by independent optimization of width for each gaussian function. Such computational procedure is able to significantly increase the agreement between experimental and theoretical spectra.
CODECS (COnvergent Distributed Environment for Computational Spectroscopy) is an interdisciplinary COST Action which aims at creating a network dedicated to computational spectroscopy, i.e. to the extraction of structural and dynamical features of molecular and supramolecular systems by in silico analysis of spectroscopic observables. The Action is organised in four Working Groups whose activities will cooperate to develop a modular, integrated computational tool for resonance, vibrational, and optical spectroscopies based on multiscale computational approaches in space and time, at quantum, semi-classical and classical levels of description of structural/dynamic molecular phenomena.