Theme I: New theoretical methods
Highly correlated systems

The fundamental methodological challenge of theoretical chemistry concerns the numerical treatment of the many-body electronic problem, which involves the representation of functions of many correlated variables and the solution of the associated equations. Recently, the DMET method (Density Matrix Embedding Theory) proposed to cut the problem of N interacting particles, into subsystems of reasonable size (easily diagonalizable) and to optimize the interaction potential of each subsystem, via a constrained self-consistent procedure based on Schmidt decomposition. If this method allows a better treatment of the electron correlation than the DFT, it does not obey a strict variational principle (Rayleigh-Ritz) which limits its implementation in the codes of quantum chemistry. From the perspective of quantum computers, which offer particularly efficient diagonalization algorithms, this method appears to be optimal for anticipating the locks related to the upcoming digital transition. 

In this context, we propose a variational alternative to the DMET method to access a better description of correlated systems, without additional numerical cost compared to the FTD (ANCRE Projects, MUSE).
Hamiltonians Diabatic

Theoretical and computational photochemistry enjoys a fairly central status because of its declination into three major challenges to quantitatively describe (i) electronic wave functions, (ii) potential energies and (iii) nuclear wave functions. If quantum dynamics have reached a maturity allowing today to consider the treatment of molecules with a few dozen atoms, a lock persists for the representation of the potential energy of large systems. A strategy based on the use of a diabatic representation of electronic states (whose chemical nature is preserved) is a way of prioritizing correlation terms in potential energy, rationally. The notion of diabatic state underlies many essential concepts in chemistry (correlation diagrams, site models, Valence-Bond approaches, Marcus theory…) which all describe chemical reactivity as a reorganization of bonds and free pairs, correlated with a change in system geometry. In this context, we wish to implement an original diabatization method, based on descriptors characterizing the locality of charge transfers or excitations, associated with excited electronic states.

These tools will make it possible to describe in a compact way the temporal evolution of the electron density during a photochemical or photophysical process, then they can be used as part of a post-processing of on-the-fly dynamics to define the relevant subspace of diabatic states explored during the evolution of the photo-excited system.
Development of Quantum Chemistry Codes

Several members of the department have significant development activity on different free computational codes, now distributed in the community of theoretical chemists to meet the specific modeling needs of the CPT&M department and the international community. The implementations concern: (1) The integration of free energy calculation and QM/MM embedding methods in the DFT code of Mon2K – density of Montreal, (2) The interfacing of the MESRA code – Molecular Electronic-Structure Reorganization: Analysis with current codes of quantum chemistry (Gaussian09, Gaussian16, Quantum Package…).(3) Implementation of on-the-fly diabatization procedures in the QUANTICS code

Theme II: Multi-scale approaches
Thermal Transport in Disordered Materials

Predicting thermal transport properties in materials with a high defect rate is a typically multi-scale problem. The possibility of quantifying these properties ab initio would be a significant advance in various fields of application such as thermoelectricity where the most efficient materials have structural/chemical disorders and/or extensive defects. If solving the Boltzmann transport equations gives reliable results for accessing the transport properties of perfect crystalline materials, this method cannot be strictly extended to amorphous/disordered materials for which the spectral response function is not known. We therefore propose to develop a new computational procedure to address this problem by integrating structural disorder, chemical disorder and inhomogeneities (dislocations, multi-phasing…) often encountered in thermoelectric materials. For each of these steps, certain theoretical tools will have to be developed (DECATRAN Project). 

The thermal conductivity can then be calculated by a multi-scale approach, by integrating the parameters from ab initio calculations into the transport equations that will be solved by Monte Carlo simulations.
Responsiveness to interfaces

Interface responsiveness is a rapidly expanding field of theoretical chemistry that is part of a more general context related to the development of multi-scale approaches. In the field of catalysis (homogeneous or heterogeneous) and energy, the control and control of interfaces is essential to improve the performance of devices currently developed on an industrial scale. Whether they involve molecular complexes interacting with a solvent, porous materials, nanoparticles subjected to a gas or solid electrodes in contact with liquid electrolytes, catalytic or electrochemical interfaces are the seat of chemical reactions that require not only a good atomistic description of the interface (molecular geometry, morphology of particles, surfaces, pore size in hybrid architectures…) but also and above all the consideration of the environment (solvent, gas …) and external stimuli (light, potential, temperature…) which can significantly modify the thermodynamic and kinetic quantities of these reactions. In this context, we will continue our previous methodological developments to integrate the complexity of all these interfaces into generalized canonical and/or Grand canonical approaches coupling quantum (DFT/ab initio), hybrid (QM/MM, QM/PCM), molecular dynamics (classical and ab initio) and Monte Carlo methods.

Quantifying the thermodynamic and kinetic parameters of catalytic and electrochemical interfaces is an essential prerequisite for statistically addressing the reaction mechanisms involved in industrial devices and contributing to the improvement of their performance.

New EVB reactive potentials

A unifying theme for the department concerns the development of new reactive potentials for classical molecular dynamics. A reliable and easy-to-implement approach to modelling chemical reactions requires a properly defined potential energy function. The definition of reactive empirical potentials makes it possible to deal with large systems, for which molecular dynamics ab initio remain prohibitive. Among the different classes of reactive potentials, we particularly want to develop EVB-type potentials, based on a “Valencia Bond” vision transposed to classical empirical force fields dependent on interatomic connectivity.Thus, in contrast to empirical reactive potentials such as ReaxFF (which presuppose a complete redefinition of the different contributions of potential energy) the EVB approach is based on the interaction of several “diabatic” valence states, for which the energy is calculated by a usual force field. The interaction between states then occurs via a coupling term, like non-adiabatic couplings in photochemistry. We propose to develop this type of reactive potentials to study the dynamics and reactivity of many systems (organometallic compounds in solution, solid-liquid / liquid-liquid / liquid-gas interfaces, electrochemical interfaces). These reactive potentials will also be used in semi-classical molecular dynamics approaches (quantum Langevin) to integrate part of the nuclear quantum effects without penalizing the computational effort (POCEMON Project). Finally we will use these same potentials for the simulation of spectroscopic properties (IR and Raman, NMR, …) integrating both anharmonicity and nuclear quantum effects.

Cross-cutting axes: Applications

The synergy Theory/Experience is a strong axis of the Department, essential to the evolution of the theoretical methods developed in axes 1 and 2. The application of these theoretical tools to concrete cases resulting from experimental work makes it possible not only to interpret the observed measures, but also to validate the methods and/or better understand their limits. The application projects initiated in the department clearly echo the experimental projects developed in the other departments of the Institute, and will therefore be an opportunity to perpetuate or initiate inter-departmental collaborations, in connection with the cross-cutting themes of the Institute (VIDICAT Projects, RS2E). 

Electrode Materials and Electrolytes for Metal-Ion Batteries

In the field of energy storage, research over the past 10 years on electrode materials has significantly increased the energy density of Li-ion batteries. The counterpart of this major scientific breakthrough is that the liquid electrolytes used in these devices now have a potential stability window lower than the battery voltage. This leads to parasitic reactions and the formation of more or less passivating layers at the interfaces (SEI) that interfere with the proper functioning of the device. Understanding the solvation and ionic transport properties in liquid or solid electrolytes developed today on an industrial scale is a typically multi-scale problem in which the nature and composition of solvents play both on the local properties of the electrolyte (solvation energy, structure of solvated species …) but also and above all on the properties of ionic transport at the meso- or macroscopic scale (ion diffusion, structure of the electrochemical double layer …). This theme builds on the developments made in Axis 2 to address realistic systems, in close collaboration with several experimental and industrial groups.

Heterogeneous catalysis

Heterogeneous catalysis is a key technology for many industrial processes – from petrochemicals to biomass valorization – for which catalytic reactivity is provided by different architectures (surfaces, nanoparticles or porous materials). The transformation of biomass into products useful for industry and respectful of the environment is a very important issue that induces a very sustained international competition for the development of new materials adapted, efficient and economically viable.The theoretical study of these catalytic processes is carried out in several stages, incorporating classical methods (molecular dynamics, Monte Carlo) to search for active sites and identify interactions between these sites, and quantum methods (DFT, ab initio, AIMD) or hybrid methods (QM/MM/solvents) to quantify these interactions taking into account the environment.When catalysis is supported by metallic surfaces or nanoparticles (NP), other parameters such as the morphology of NP, the nature and type of surfaces and their coverage rates must also be taken into account to access a good description of the catalytic process and deduce “universal” trends. Mastering the complexity of the catalytic material to correctly describe the solid/gas and solid/solvent interface therefore represents a double challenge for the theorist who must take into account in his atomistic simulations not only the variety of surface sites (defects, acid-base Lewis and Brønsted sites etc …) and species present on the surface (co-adsorbates, traces of water etc …) but also the experimental conditions (temperature, gas pressure etc …).

Spectroscopic methods

The simulation and theoretical analysis of spectroscopic measurements is both a source of validation of the methods used but also provides information on the nature of the species studied (structure, electronic state, etc.). and the impact of their environment. It is therefore important to be able to push the reproduction of spectral information as far as possible, by combining the necessary methods (the most advanced electronic methods taking into account the size and nature of the species, coupling with dynamic methods when necessary). In addition, an effort must also be made on the interpretation of the results. This last point is particularly important for the dialogue between experience and theory. We will continue the studies already initiated in the context of NMR, IR-Raman and XAS spectroscopy, integrating the new methodological developments proposed in axes 1 and 2 to (i) relate the magnetic anisotropy properties of an atom to the nature of its chemical bonds and the catalytic properties of transition metal complexes, (ii) elucidate the mechanisms of electrochemical insertion using the vibrational response (Raman) of disordered electrode materials (VASELINA Project), (iii) validate the reducing coupling mechanism responsible for anionic redox in high energy density electrode materials (RS2E Project) and (iv) correlate IR intensities to phase transitions in low crystallinity organic polymers.