Research

1. Simulation of Strong Light-Matter Interaction in Molecules and Materials

   In recent years strong light-matter coupling emerged as a completely new approach to control and tune, with unprecedent accuracy, the properties of matter and electromagnetic fields, representing also a testing bed for prediction of new quantum mechanics effects.
   Recent experimental works demonstrated, for instance, that strong light matter coupling inside an optical cavity can be used to manipulate chemical reactivity of molecules or change electronic properties of solid state materials.
   Part of my research is then dedicated to the development of new theoretical ab-initio methodologies to describe electron-photon correlation in hybrid light-matter systems.
   In our first work on this topic, we extended the well established Bethe-Salpeter formalism to describe excitons coupled to a quantum field in a cavity. The study has been performed on Transition Metal Dichalcogenides (TMDs) monolayers, systems characterized by very large binding energy excitons. In this work we demonstrated that the coupling to the electromagnetic field is able to break standard selection rules allowing for the population and identification of dark states usually inaccessible to standard spectroscopic techniques. In this work the effects produced by dielectric spacings and multi-layer systems have been also investigated.

   

   Relevant Publications

    

   “Ab-initio Exciton-polaritons: Cavity control of Dark Excitons in two dimensional Materials” S. Latini, E. Ronca, U. De Giovannini, H. Hübener, A. Rubio, arXiv,arXiv:1810.02672, 2018.

    

2. Simulation of Spectra in Strongly Correlated Systems

   Electronic spectra represent the main source of information to investigate photochemical and electrochemical functions of materials, but computing them with high accuracy has been always a significant challenge for standard numerical techniques.
   Part of my research is dedicated to the extension of a powerful numerical technique, the density matrix renormalization group (DMRG), to the time-domain, so as to calculate the electronic spectra of realistic materials. In one of the first applications I showed – for the very first time – that quasi-exact spectra of jellium, the most fundamental material model, can be computed. These benchmark calculations further allowed me to assess the accuracy of conventional approximations (such as the GW method) that have been the workhorses of the past, with some quite surprising findings. The accuracy achieved in our work sets the stage for future projects, focused to address systems and questions fundamental to both chemistry and physics – predicting from first principles the X-ray absorption spectra of metalloenzyme clusters, as relevant to chemical biology; determining ab-initio the Kondo spectrum for metal adsorbates, as relevant to condensed matter physics; and simulating in realtime electron transfer in photovoltaic materials, as relevant to materials science and the energy industry.

   

   Relevant Publications

   “Time-step targeting time-dependent and dynamical density matrix renormalization group algorithms with ab initio Hamiltonians” E. Ronca, Z. Li, C. A. Jimenez-Hoyos, G. K.-L. Chan, J. Chem. Theory Comput., 13 (11), 5560–5571, 2017.

   “Spectral Functions of the Uniform Electron Gas via Coupled-Cluster Theory and Comparison to the GW and Related Approximations“, J. McClain, J. Lischner, T. Watson, D. A. Matthews, E. Ronca, S. G. Louie, T. C. Berkelbach, and G. K.-L. Chan, Phys. Rev. B, 93 (23), 235139, 2016

3. Charge Transfer Effects in Molecular Systems and Materials

   A deep comprehension of Charge Transfer (CT) processes is of fundamental importance to fully understand several physical, chemical and biological phenomena. However, the study of CT is complicated by the absence of experimental techniques able to directly detect charge movements inside a molecular system. For this reason the use of theoretical methodologies represents one of the main sources of information for this kind of processes.
   Part of my research is dedicated to the development and application of new theoretical methodologies to investigate CT processes in material and molecular systems both in the ground and in the excited state.

   • Study of Charge Transfer during the formation of a Chemical Bond

     The key approach I use to investigate CT occurring during a chemical bond formation is the Charge Displacement (CD) analysis. This method, developed in Perugia inside the group of Prof. Francesco Tarantelli, is able, differently from the commonly used strategies, to provide quantitative information about charge rearrangements without resorting to orbital localization schemes.

     

     The method is based on the definition of an axis (z) of interest (typically one joining the interacting species) and a variation in the electron density (Δρ) taking place upon formation of the intermolecular complex (usually the density difference between the complex and the isolated noninteracting partners placed at the same positions they occupy in the complex). The Charge Displacement function Δq measures, at each point along the z axis, the electron charge that, upon formation of the adduct, has crossed from the right to the left, the plane through z, perpendicular to the axis. If the Δq function never crosses zero in the region dividing the fragments, a net CT between the interacting species is occurring. If a plane can reasonably be taken as separating the fragments, it is possible to obtain an estimate of the amount of charge that has been transferred from one system to the other.

     This approach is able, as observed for weakly bound complexes containing water molecules, to provide a clear picture of the charge rearrangements occurring during the bond formation. Simple electron delocalization models demonstrated also that the amount of CT estimated by the CD analysis in able to produce a bond stabilization in line with that mesured by accurate molecular-beam scattering experiments.

     
     The CD approach is also effective in the description of charge rearrangements occurring in more complex systems like those used in the field of hybrid photovoltaics. The combination of simple electrostatic models with the CD analysis has been able to quantify the effects produced, in a Dye-Sensitized Solar Cell for example, by the adsorption of a certain dye-sensitizer on the electronic properties of the semiconductor surface.

     

     The CD approach has been also applied to the study of new generation lead-halides perovskite-based devices revealing interesting interpretations to still hidden effects produced by the inclusion of different halides during the material synthesis.

     

     Relevant Publications

     “Revealing charge-transfer effects in gas-phase water chemistry“, D. Cappelletti, E. Ronca, L. Belpassi, F. Tarantelli, F. Pirani, Acc. Chem. Res., 45 (9), 1571-1580, 2012

     “A Quantitative View of Charge Transfer in the Hydrogen Bond: The Water Dimer Case“, E. Ronca*, L. Belpassi, F. Tarantelli, ChemPhysChem, 15 (13), 2682-2687, 2014

     “Influence of the dye molecular structure on the TiO₂ conduction band in dye-sensitized solar cells: disentangling charge transfer and electrostatic effects“, E. Ronca, M. Pastore, L. Belpassi, F. Tarantelli, F. De Angelis, Energy Environ. Sci., 6 (1), 183-193, 2013

     “First-principles investigation of the TiO₂/organohalide perovskites interface: The role of interfacial chlorine“, E. Mosconi, E. Ronca, F. De Angelis, J. Phys. Chem. Lett., 5 (15), 2619-2625, 2014
     

   • Study of Charge Transfer at the Excited State

     It results clear that the CD approach, analysed in the previous section, can be easily extended to the study of any other physical and chemical phenomena producing a change in the electron density of the system. An interesting case is for instance the study of electron density modifications  happening in a material exposed to the electromagnetic radiation. In order to study these interesting processes I formulated an Excited State variant of the CD approach (ESCD).
     

     Relevant Publications

     “Charge-displacement analysis for excited states“, E. Ronca*, M. Pastore, L. Belpassi, F. De Angelis, C. Angeli, R. Cimiraglia, F. Tarantelli, J. Chem. Phys., 140 (5), 054110, 2014

   • Relaxed Density- Multi Reference Perturbation Theory (RD-MRPT)

     Inspired by the results obtained from the ESCD analysis I developed the Relaxed-Density Multi Reference Perturbation theory (RD-MRPT), a new method able to correct Time Dependent Density Functional Theory (TDDFT) CT excitation energies at a reasonable computational cost. This method uses the relaxed density, obtained from TDDFT, to build a set of Natural Orbitals (NOs) able to accurately represent the Excited State wave-function just as a CI expansion of few determinants. Including dynamical correlation by second order perturbation theory (in our case by N-Electron Valence Perturbation theory) starting from this zeroth order wave-function, accurate excitation energies for a significant set of systems can be obtained.

     
     The approach has been tested on valence, ionic and CT excited states of some small and medium-sized prototype molecules showing always accurate results comparable to those obtained by much more demanding computational approaches. In the future this method, circumverting the onerous CAS/RASSCF orbital optimization step in a MRPT calculation could lead to accurate description of excited states of larger systems of high applicative interest.

     Relevant Publications

     “Density Relaxation in Time-Dependent Density Functional Theory: Combining Relaxed Density Natural Orbitals and Multireference Perturbation Theories for an Improved Description of Excited States“, E. Ronca*, C. Angeli, L. Belpassi, F. De Angelis, F. Tarantelli, M. Pastore, J. Chem. Theory Comput., 10 (9), 4014-4024, 2014

   • Kinetic of Charge Transfer in Hybrid Photovoltaic Systems

     Finding strategies to maximize the efficiency of a solar cell not only requires an optimization of the amount of injected charge, but it is extremely important also verifying that the injection, and the following electron transport, is as fast as possible. In this perspective a part of the research activity is dedicated also to the study of the charge injection kinetic, in the particular field of hybrid photovoltaics. To perform this analysis I used two of the most established theoretical approaches commonly applied to provide estimations of the injection rates (Newns-Anderson and a “quasi-diabatic” approach developed by Thoss et al.). The results obtained by this study allowed for an accurate quantification of the effects produced by structural modifications in the sensitizer and by the adsorption of cations (actually present in a DSC device) onto the semiconductor surface on the charge injection rate and consequently on the short-circuit current of the cell. The estimated injection rates resulted in excellent agreement with those measured in devices built under the same conditions.

     Relevant Publications

     “Effect of sensitizer structure and TiO₂ protonation on charge generation in dye-sensitized solar cells“, E. Ronca, G. Marotta, M. Pastore, F. De Angelis, J. Phys. Chem. C, 118 (30), 16927-16940, 2014

4. Spin-Orbit Effects in Hybrid Photovoltaics

   A part of my research is deticated also to the study of the effects produced by relativity on the optical and structural properties of materials involved in hybrid photovoltaic devices.
   Aa a first analysis I quantified the effects produced by spin-orbit coupling on the absorption spectrum of dyes containing heavy atoms (Ru, Os, etc) commonly applied in DSC devices. From this study I was able, for a wide series of sensitizers, to characterize all the transitions, both singlet-singlet and singlet triplet, present in the lower energy region of the spectrum. The analysis revealed that only in the presence of the heavier metals, such us Os, the lower energy transitions can be attributed to singlet-triplet transition allowed by the strong spin-orbit coupling. When lighter metals (i.e. Ru) are present, extensions of the absorption spectrum in the near IR region are instead attributable to electronic structure modifications produced by the ligands.

   
   A detailed relativistic study has been performed also to explain the structural properties of lead-halides perovskites, materials that, in the last years, are revolutionizing the landscape of emerging photovoltaic technologies. Because of the presence of Pb the inclusion of relativistic effects and in particular of spin-orbit coupling is mandatory in order to accurately describe the electronic structure of these materials. The performed analysis revealed that changing the structure of the perovskite also the electronic structure significantly change and the importance of relativistic effects in the description of the material vary with the covalency of the involved bonds. The described analysis allowed also for the identification of the most stable geometry, property difficult to extract from experimental data.

   Relevant Publications

   “Time-Dependent Density Functional Theory Modeling of Spin–Orbit Coupling in Ruthenium and Osmium Solar Cell Sensitizers“, E. Ronca, F. De Angelis, S. Fantacci, J. Phys. Chem. C, 118 (30), 17067-17078, 2014

   “Impact of spin–orbit coupling on photocurrent generation in ruthenium dye-sensitized solar cells“, S. Fantacci, E. Ronca, F. De Angelis, J. Phys. Chem. Lett., 5 (2), 375-380, 2014

   “Cation-induced band-gap tuning in organohalide perovskites: interplay of spin–orbit coupling and octahedra tilting“, A. Amat, E. Mosconi, E. Ronca, C. Quarti, P. Umari, M.K. Nazeeruddin, M. Grätzel, F. De Angelis, Nano Lett., 14 (6), 3608-3616, 2014