Symmetrized Drude Oscillator Force Fields Improve Numerical Performance of Polarizable Molecular Dynamics

Drude oscillator potentials are a popular and computationally efficient class of polarizable models that represent each polarizable atom as a positively charged Drude core harmonically bound to a negatively charged Drude shell. We show that existing force fields that place all non-Coulomb forces on the Drude core and none on the shell inadvertently couple the dipole to non-Coulombic forces. This introduces errors where interactions with neutral particles can erroneously induce atomic polarization, leading to spurious polarizations in the absence of an electric field and exacerbating violations of equipartition in the employed Carr-Parinello scheme. A suitable symmetrization of the interaction potential that correctly splits the force between the Drude core and shell can correct this shortcoming, improving the stability and numerical performance of Drude oscillator based simulations. The symmetrization procedure is straightforward and only requires the rescaling of a few force field parameters

Design and development of solid-state functional materials for Na-ion batteries

This Thesis addresses new functional materials for Na-ion battery (NIB) applications. Since the breakthrough of Li-ion battery (LIB), extensive research has been focusing on alternatives to Lithium, based on cheaper and widespread elements for sustainable energy storage solutions. In this context, the effective large-scale deployment of NIB requires great efforts in the development of good Na+ host anodes, high-energy cathodes and safe electrolytes. New components must ensure enhanced efficiency in the NIB operating processes (i.e., Na+ insertion/extraction at the electrode/electrolyte interface and Na+ transport through the electrolyte) for empowering high energy density and long-term cycle stability. Here, we present NIB materials optimization through an innovative approach, based on computational methods that are directly related to experiments. Our aim is to unveil the most important features that can affect the material capabilities towards Na+ uptake, transport and storage. During the research activity at Università di Napoli Federico II, state-of-the-art DFT methods have been employed to investigate the structure-property relationship of solid-state nanoelectrodes. Our studies on TiO2 anatase and MoS2/graphene 2D-heterostructure reveals that sodiation mechanisms are driven by intrinsic structural features. Migration barriers are directly correlated to structure-dependent descriptors, such as the accessible area for the intercalating Na+ at TiO2 surfaces, and the S coordination around the migrating Na+ within MoS2/graphene interface. From these outcomes, we provide new design strategies to improve the electrode efficiency upon sodiation, for example suggesting the preferential growth of TiO2 along the (001) direction or the introduction of S vacancies in MoS2 monolayers. On the cathode side, we unveil the charge compensation mechanism occurring in NaxNi0.25Mn0.68O2 upon desodiation, with a major focus on the O-redox chemistry at very low Na loads. Molecular O2 is predicted to be released from Mn-deficient sites in the bulk cathode via formation of superoxo-species and preferential breaking of labile Ni-O bonds. We prove that increasing M-O covalency via suitable doping would prevent O2 loss and allows to fully recover a reversible process. Research stages at ENS de Lyon and the R&D laboratory of Lithops s.r.l. have been dedicated to the optimization of electrolyte materials. By development and application of polarizable force fields in molecular dynamics simulations, we report reliable predictions of Na+ diffusion and solvation properties into the PyrFSI room-temperature ionic liquid (RT-IL). We combine RT-ILs with cross-linked PEO matrix to obtain highly conductive polymeric membranes. Galvanostatic cycling of Na metal based cells containing these innovative polymer electrolytes and state-of-the-art electrodes shows promising performances and paves the route to further assessment of efficient cells. The foreseen integration of these studies will provide new understanding on the complex charge transfer processes occurring at the electrode/electrolyte interface during battery functioning. The new knowledge on electrochemical behavior of advanced materials will be key for boosting the NIB technology in the near future

Towards Improving The Accuracy of Implicit Solvent Models and Understanding Electrostatic Catalysis in Complex Solvent Environment

This thesis develops improved protocols for studying reactions in solution and uses them to explore the possibility of harnessing complex non-standard solvent environments to catalyse chemical reactions. The thesis covers different but related topics: Improving the accuracy of implicit solvent models. Implicit solvent models are simple cost-effective strategies for modelling solvent as a polarizable continuum. However, the accuracy of this approach can be quite variable. Herein, we examine approaches to improving their accuracy through cavity scaling, the choice of theoretical level and the inclusion of explicit solvent molecules. For SMD, we show that the best performance is achieved when cavity scaling is not employed, while for PCM we present a series of electrostatic scale factors that are radii, solvent and ion type dependent. For both families of method, we also highlight the importance choosing an appropriate level of theory, and identify when explicit solvent molecules are required.. Modelling electrostatic catalysis in complex solvent environment. Recent nanoscale experiments have shown that electric fields are capable of catalysing and controlling chemical reactions, but experimental platforms for scaling these effects remain elusive. Herein, two different approaches to addressing this challenge are explored. The first is using the internal electric field of ordered solvents and ionic liquids, the second is using the electric fields that form naturally at the gas-water interface. A multi-scale modelling approach was developed using polarizable force field based molecular dynamic simulation, post-HF, DFT and semi-empirical quantum chemical calculations. We showed that after exposure to an external electric field, ensembles of solvent or ionic liquid molecules become ordered and this ordering can generate an internal electric field, which persists even after the external potential is removed. Experimental collaborators subsequently detected this field as an open-circuit potential that is strong and long-lived. Computationally we showed that this field is enough to lower reaction barriers by as much as 20 kcal mol-1, and we also developed a predictive structure-reactivity model to choose ionic liquids that optimize this field. In the second approach, we harnessed the electric fields of the gas-water interface. A collaborator showed that in the presence of static, inert gas bubbles, the oxidation potential of HO anion/HO radical was dramatically lowered (by more than 0.5V), much more than any subtle concentration effects predicted by the Nernst equation. Further experiments showed that a high unbalanced concentration of HO- ions (as much as 5M) accumulate at the interface. Our multi-scale modelling calculations showed that this reduction in potential was due to the mutual repulsion of the HO- ions and as little as 1M unbalanced excess was enough to explain the experimental results. The work raises opportunities in reducing the cost of electrochemical processes, and points to electrostatic effects contributing to the well-known catalytic effects of "on water" reactions. Works in this thesis are expected to be useful in the future studies of solution-phase pKa, redox potential, electrostatic catalysis and ionic liquids-based electrochemical devices

Marco de trabajo termodinámico integrado para la absorción de refrigerantes fluorados en líquidos iónicos

El sector de la refrigeración y aire acondicionado tiene un elevado impacto ambiental ocasionado por las emisiones indirectas asociadas al consumo de energía de los equipos de refrigeración, así como a las emisiones directas de los gases refrigerantes de efecto invernadero. Esta tesis está consagrada al desarrollo de un marco de trabajo que combina estudios experimentales, modelado matemático y herramientas computacionales novedosas destinado a la selección de líquidos iónicos que proporcionen las características termodinámicas más adecuadas para su uso como disolventes de hidrofluorocarbonos e hidrofluoroolefinas en dos tipos de aplicaciones: i) sistemas de refrigeración por absorción con eficiencia energética mejorada, y ii) destilaciones extractivas para separar mezclas de refrigerantes obtenidas a partir de dispositivos al final de su vida útil, y recuperar los gases con bajo potencial de calentamiento atmosférico para su reutilización. La presente tesis doctoral contribuye a la evolución del sector de la refrigeración hacia la economía circular y propone las herramientas necesarias para el desarrollo de procesos que faciliten esta transición y la mitigación de los efectos del cambio climático.The refrigeration and air conditioning sector has an elevated environmental impact resulting from the indirect emissions derived of its energy consumption, and from the direct emissions of GWP hydrofluorocarbons from equipment at its end of life. This thesis develops an integrated framework that combines experimental studies, mathematical modeling, and novel computational tools for the selection of ionic liquids with the adequate thermodynamic properties for their use as solvents of hydrofluorocarbons and hydrofluoroolefins in two applications: i) absorption refrigeration systems that increase the efficiency of the refrigeration devices, and ii) extractive distillations aimed to separate azeotropic and close-boiling-point mixtures of fluorinated gases, with the goal of recovering low-GWP refrigerants from end-of-life equipment. This doctoral thesis contributes to the evolution of the refrigeration sector towards the circular economy and proposes the necessary tools for the development of processes that facilitate this transition, as well as the mitigation of the effects of climate change.Además, la investigación de esta tesis ha sido parcialmente financiada por el Fondo Europeo de Desarrollo Regional en el marco del programa Interreg-Sudoe a través del proyecto KET4F-Gas-SOE2/P1/P0823 “KET4F-GAS: Reducción del impacto ambiental de los gases fluorados en el espacio SUDOE mediante tecnologías facilitadoras esenciales” y por el Ministerio de Ciencia e Innovación a través de la Agencia Estatal de Investigación (MCIN/AEI/10.1039/501100011033) en el marco del proyecto PID2019-105827RB-I00 “Funcionalización de membranas como elemento clave en el desarrollo de procesos avanzados de separación”, correspondiente a la convocatoria de 2019 «Proyectos de I+D+i», en el marco del Programa Estatal de Generación de Conocimiento y Fortalecimiento Científico y Tecnológico del Sistema de I+D+i Orientada a los Retos de la Sociedad, del Pla Estatal de Investigación Científica y Técnica y de Innovación 2017-2020

Thermalized Drude Oscillators with the LAMMPS Molecular Dynamics Simulator.

International audienceLAMMPS is a very customizable molecular dynamics simulation software, which can be used to simulate a large diversity of systems. We introduce a new package for simulation of polarizable systems with LAMMPS using thermalized Drude oscillators. The implemented functionalities are described and are illustrated by examples. The implementation was validated by comparing simulation results with published data and using a reference software. Computational performance is also analyzed

Molecular Dynamics Simulation

Condensed matter systems, ranging from simple fluids and solids to complex multicomponent materials and even biological matter, are governed by well understood laws of physics, within the formal theoretical framework of quantum theory and statistical mechanics. On the relevant scales of length and time, the appropriate ‘first-principles’ description needs only the Schroedinger equation together with Gibbs averaging over the relevant statistical ensemble. However, this program cannot be carried out straightforwardly—dealing with electron correlations is still a challenge for the methods of quantum chemistry. Similarly, standard statistical mechanics makes precise explicit statements only on the properties of systems for which the many-body problem can be effectively reduced to one of independent particles or quasi-particles. [...