Photochromic molecules are increasingly used in the design of innovative functional materials with applications in nanosciences, biology, and photonic devices. These compounds convert photonic energy into chemical energy on an ultrafast (picosecond) timescale to bring about a reversible transformation between two isomers. The change in electronic and molecular structures following electronic excitation results in a change of physical properties, which forms the basis of the applications listed above. The majority of the investigated photochromic compounds relies on the use of organic molecules such as azobenzenes or dithienylethenes. A photochromic system with remarkable properties, yet not as well known, is the dimethyldihydropyrene (DHP) molecule. Indeed, DHP can be quantitatively and reversibly isomerized into cyclophanediene (CPD) under visible light, but has also the singular property to be able to produce, store and release singlet oxygen (1O2) upon irradiation at low energy under aerobic conditions. 1O2 is of major interest particularly for medical purposes. Other photochromic compounds based metal complexes have also been discovered. This is the case of ruthenium nitrosyl complexes (Ru-NO), which can be photoisomerized in their isonitrosyl form (Ru-ON). These complexes also have the remarkable property to have the capability to photorelease nitric oxide (NO•), which plays an important role in many physiological mechanisms. It thus appears that DHP and Ru-NO derivatives display remarkable and complementary properties. However, these compounds have not been much exploited yet and their underlying photoswitching mechanism and releasing capability of biologically active species remain poorly understood. The PHOTOCHROMICS project plans to intimately associate DHP and Ru-NO units in order to design hybrid DHP-Ru-NO multifunctional compounds. The objectives are the design of i) multiphotochromic compunds and ii) bimodal molecular systems capable of producing simultaneously 1O2 and NO• by photoinduced stimuli. Indeed, the design of switchable molecular architectures that can exist in several stable states is a growing field. Similarly, the possibility to produce at the same time the 1O2 and NO• biologically active species, which both display remarkable antibacterial properties, is of tremendous interest in the medical field, as synergistic effects have already been observed. This proposal gathers theoretical and experimental chemists. Original hybrid compounds will be synthesized using technics in organic synthesis and coordination chemistry. The properties of the different states involved in the multiphotochromic systems and the production / activation of 1O2 and NO• will be analyzed by spectroscopic, photophysical, photochemical and electrochemical studies. A major task will be the rationalization of the underlying mechanisms involved. We will use advanced computational methodologies beyond the current state-of-the-art, which will allow static and dynamic investigations of these systems in their environment. The main originality of this proposal lies in the remarkable properties and complementarity of the DHP and Ru-NO derivatives, whether in the field of photochromism or in the activation of small molecules of biological interest. To merge these two units in hybrid systems represent a formidable opportunity, which constitutes the heart of this project. The PHOTOCHROMICS proposal is a fundamental research project whose expected results will bring significant progress in the understanding of the involved systems. However, if the end results proved to be full of promise, the designed compounds might be exploited for application purposes in the future.

In this project we confront one of the grand challenges of materials science and condensed matter physics: the development of predictive and reliable approaches to describe and understand materials and ultimately to predict new ones of strategic technological interest. A unique source of information about electronic structure and excitations in materials is photoemission. In this project we propose to develop an original strategy for getting access to such spectra, even in situations where standard approaches fail. This will be achieved by combining in an innovative way Many-Body Perturbation Theory (MBPT) and Quantum Monte Carlo (QMC). On the experimental side, modern synchrotron sources can provide detailed insight on photoemission spectra, thanks to their high intensity and broad photon energy range. However, the interpretation of the experimental data is far from obvious, and theory represents an essential complementary tool. In particular, so-called first-principles methods, such as Density Functional Theory and MBPT based on Green’s functions, promise to be predictive, since no adjustable parameters are involved. However, standard implementations of these methods are known to work reasonably well for weakly to moderately correlated materials, such as metals and standard semiconductors (e.g., Si or GaAs) but to fail for most strongly correlated systems, which are of paramount importance both from the scientific and technological point of view. A paradigmatic example of this kind of materials is paramagnetic NiO, which is erroneously predicted to be a metal by standard approximations. This of course sets limits to the description and prediction of metal-insulator phase transitions. Based on our recent developments, we propose here to follow a different and original route based on expressing the PES in terms of n-body density matrices, which we refer to as the Many-body Effective Energy Theory (MEET). Preliminary results on bulk NiO give a qualitatively correct picture, however it is clear that further improvements are necessary. To achieve this we need more accurate density matrices, which we propose to obtain from QMC. The project can be divide in two main parts: 1) Implementation and calculation of the n-body density matrices in QMC and use in the MEET. This step will tell us up to which n-body density matrix we need to have accurate PES. We will first test the MEET+QMC paradigm on simple metals and standard semiconductors, and then we will attack strongly correlated systems, bulk NiO being the first target. This latter task requires the use of a multi-determinant wavefunction, which is a quite unexplored and challenging field for solids in QMC. We will hence use the very latest developments done in our groups. The ideal platform for this part is the open-source code QMCPACK, probably the most versatile and efficient QMC program for periodic solids. 2) The next step is to make the calculations of the PES more efficient, by using only effective 1-body and 2-body density matrices. This part requires novel formal developments, which will be done along two lines: i) approximate resummation of higher-order density matrices by using a proper terminating function; ii) using a model Hamiltonian that includes already some effective screening and deriving the corresponding 1- and 2-body density matrices to be used in the MEET. It can be expected that the project will initiate further studies, such as: i) the study of phase transitions, ii) the development of better approximations to the exchange-correlation potentials of Density Functional Theory and Reduced Density-Matrix functional theory, iii) the extension of the same strategy to the calculation of other material properties, such as absorption and electron energy loss spectroscopy within QMC.

Processes related to electronically excited states are central in chemistry, physics, and biology, playing a key role in ubiquitous processes such as photochemistry, catalysis, and solar cell technology. However, defining an effective method that reliably provides accurate excited-state energies remains a major challenge in theoretical chemistry. In CACO, we aim at developing a totally novel approach to obtain excited-state energies and wave functions in molecular systems thanks to the properties of non-Hermitian Hamiltonians. Our key idea is to perform an analytic continuation of conventional computational chemistry methods. Indeed, through the complex plane, ground and excited states can be naturally connected. In a non-Hermitian complex picture, the energy levels are sheets of a more complicated topological manifold called Riemann surface and they are smooth and continuous analytic continuation of one another. CACO's main goal is to develop a new theoretical approach allowing to connect, through the complex plane, electronic states. Instead of Hermitian Hamiltonians, we propose to use a more general class of Hamiltonians which have the property of being PT-symmetric, i.e., invariant with respect to combined parity reflection P and time reversal T. This weaker condition ensures a real energy spectrum in unbroken PT-symmetric regions. PT-symmetric Hamiltonians can be seen as analytic continuation of conventional Hermitian Hamiltonians. Using PT-symmetric quantum theory, an Hermitian Hamiltonian can be analytically continued into the complex plane, becoming non-Hermitian in the process and exposing the fundamental topology of eigenstates. Our gateway between ground and excited states are provided by exceptional points, the non-Hermitian analogs of conical intersections, which lie at the boundary between broken and unbroken PT-symmetric regions.

Low energy electrons (LEEs), a few eV, are abundantly produced during the irradiation of matter by ionizing radiation, without any precise knowledge of the mechanisms of their formation and relaxation in the environment. We will study by a coupled experimental/theoretical approach the photo-ionization of small molecules (diazabicyclo[2.2;2]-octane), in particular of biological interest (amino acids, DNA nucleobases), deposited on nanometric aggregates. The objective is to understand electron scattering at the molecular scale on time scales ranging from femto- to picoseconds. Experimentally, the Velocity Map Imaging (P3, ISMO, Orsay) will give access to angle and energy distribution of photoelectrons (PAED) emitted by the aggregates, allowing to characterize the elastic and inelastic scattering processes. In order to understand the role of the intensity of the interactions between the medium and the LEEs, different scattering environments will be tested (atomic Argon aggregates, molecular aggregates of H2O, NH3, CO2). These experimental data will provide a valuable test to validate new simulation approaches. To tackle the challenge of simulating the relaxation dynamics of photoelectrons within nanoscale aggregates, we will develop novel algorithms based on orbital-free DFT (OF-DFT). We will couple this approach to the Kohn-Sham DFT (P1, ICP, Orsay and P4, IDRIS, Orsay), to the DFTB (Tight Binding approach to DFT, P2, LCPQ, Toulouse) and to a polarizable MM (Molecular Mechanics) potential (P1, P2 and P4). These three methodological developments will allow complementary descriptions: a better quality description on a few trajectories with the OF-DFT approach, the exhaustive simulations necessary for the interpretation of the experiments being performed with more efficient but less precise methods (OF-DFTB and OF-MM). Once validated, the new algorithms will be made available to the scientific community in the Mon2k and MonNano codes. The experimentally studied systems will be simulated to predict their structures and dynamics. We will simulate the photo-ionization in time-dependent DFT, and the relaxation and diffusion phases will be simulated with the new algorithms. The experimentally and theoretically obtained PAEDs will be compared, which should lead to an advanced understanding of the evolution of the systems. We will provide the community with characteristic electron scattering times and mean free paths for the studied systems that can be used, for example, as parameters in Monte Carlo scattering codes. Thanks to BIRD we will have unique tools to characterize the impact of LEEs scattering in complex molecular systems in various contexts: biology (DNA lesions or other biomolecules), astrochemistry (molecule formation mechanisms in then interstellar medium or in planetary atmospheres/ionospheres) or space and nuclear industry.

In-depth knowledge of the consequences of interaction of biomolecules with ionizing radiations is of fundamental importance for progress in medicine, radiotherapy and radioprotection as well as for addressing the panspermia hypothesis. The physicochemical events taking place during the first picoseconds after irradiation have so far escaped the scrutiny of researchers. Yet the plethora of vibrationally hot, electronically excited and highly reactive species formed upon irradiation is likely to trigger rich and non-conventional chemistry. For complex biostructures such as DNA or proteins irradiation could undergo currently unknown chemical damages with dramatic consequences for health. In the field of astrochemistry, ionizing radiations could have played a crucial role. The bombardment of the early Earth by comets that have captured organic rich interstellar icy grains is thought to have provided the molecular precursors at the origin of life on Earth. Such an assumption depends on the survival of these precursor under cosmic irradiations. We will thus explore the early steps of radiation induced damages on biomolecules from first-principles atomistic simulations. RUBI will result in innovative simulation techniques available for the community and is expected to have a great impact on the fundamental understanding of the early steps following matter irradiation, a hot topic in many different research fields. The chemical physics following irradiation of matter produces a plethora of extremely aggressive and unstable chemical species that may directly damage molecules. The reactivity within the ultrafast (