arXiv subjects

Guillaume Jeanmairet

Publications and source records attributed to Guillaume Jeanmairet.

At least 19 recordsLinked to original sources

arXiv2026

Competing Ring-Opening and Hofmann Elimination Pathways in Aqueous TEMPO Catholytes: A First-Principles Study

Aqueous redox-flow batteries based on TEMPO derivatives are promising for large-scale energy storage, but their practical use is limited by the chemical instability of the oxidized N -oxoammonium state. In this work, we investigate the degradation of five TEMPO derivatives using ab initio molecular dynamics combined with enhanced sampling. Two proposed degradation mechanisms, ring opening and Hofmann elimination, are examined and their corresponding activation free energies are compared. For all derivatives considered, ring opening exhibits a lower activation free energy than Hofmann elimination, identifying it as the kinetically preferred degradation pathway. The magnitude of the ring-opening barrier, however, varies significantly between molecules, showing that different functionalizations strongly influence its stability toward degradation. The predicted preference for ring opening is consistent with available experimental studies, which have identified or inferred ring-opening degradation for several TEMPO-based catholytes. These results provide an atomistic picture of degradation pathways that are difficult to resolve experimentally and highlight the importance of molecular structure in controlling the kinetic stability of TEMPO derivatives in aqueous electrolytes.

physics.chem-ph↗

arXiv2026

The Angular Localization Function (ALF): a practical tool to measure solvent angular order with Molecular Density Functional Theory

Molecular density functional theory is a powerful technique for efficiently computing the spatially and orientationally dependent equilibrium density of a molecular solvent around an arbitrary solute. This density encodes the detailed solvent structure, but contains so much information that it is difficult to interpret comprehensively. Although spatial dependence is frequently analyzed through orientationally integrated number density, angular information remains poorly exploited. The present work addresses this gap by introducing a function that provides a local measure of the angular order: the Angular Localization Function (ALF), derived from the ideal free energy functional, which quantifies the entropy. We discuss the connections between ALF and well known statistical functions. We illustrate the utility of ALF by discussing the solvent structure for three systems immersed in water: water as a solute, an octanol molecule, and three clay minerals (talc, fluorotalc and pyrophyllite) with small differences in their structure leading to subtle effects on their interactions with water. ALF provides information complementary to quantities such as the average polarization or charge density to characterize the local orientational distribution of solvent molecules around solutes and next to surfaces. It also offers a convenient visualization tool akin to the Electronic Localization Function (ELF) used to analyze bonding in quantum chemistry.

physics.chem-ph↗

arXiv2026

A variational formulation of the free energy of mixed quantum-classical systems: coupling classical and electronic density functional theories

Combining classical density functional theory (cDFT) with quantum mechanics (QM) methods offers a computationally efficient alternative to traditional QM/molecular mechanics (MM) approaches for modeling mixed quantum-classical systems at finite temperatures. However, both QM/MM and QM/cDFT rely on somewhat ambiguous approximations, the two major ones being: i) the definition of the QM and MM regions as well as the description of their coupling, and ii) the choice of the methods and levels of approximation made to describe each region. This paper addresses the second point and develop an exact theoretical framework that allows us to clarify the approximations involved in the QM/cDFT formulation. We establish a comprehensive density functional theory (DFT) framework for mixed quantum-classical systems within the canonical ensemble. We start by recalling the expression of the adiabatic equilibrium density matrix for a mixed system made of Nqm quantum and Nmm classical particles. Then, we propose a variational formulation of the Helmholtz free energy in terms of the full, non-equilibrium, QM/MM density matrix. Taking advantage of permutational symmetry and thanks to constrained-search methods, we reformulate the computation of the Helmholtz free energy using only the quantum and classical one-body densities.This paper generalizes both cDFT and electronic DFT (eDFT) to QM/MM systems. We then reformulate the functional to make the standard eDFT and cDFT Levy-Lieb functionals explicitly appear, together with a new universal correlation functional for QM/MM systems. A mean-field approximation is finally introduced in the context of solvation problems and we discuss its connection with several existing mixed cDFT-eDFT schemes. An extension to the semi-grand canonical ensemble, where the number of classical particles is allowed to fluctuate, is provided in the supplementary materials.

cond-mat.stat-mech↗

arXiv2025

Prediction of the aqueous redox properties of functionalized quinones using a new QM/MM variational formulation

We recently proposed a method coupling quantum mechanics (QM) methods and molecular density functional theory (MDFT) to describe mixed quantum-classical systems [J. Chem. Phys. 161, 014113 (2024)]. This approach is particularly appropriate to account for solvent effect into QM calculations. We introduce a new variational formulation for the grand potential of a mixed quantum-classical system. Within the Born-Oppenheimer approximation and neglecting electronic entropy, the quantum solute is described by a product of electronic and nuclear density matrices, both depending parametrically on coordinates of the classical solvent. It can then be shown that a functional of the total density matrix satisfies a variational principle for the grand potential. Using a mean-field approximation, we express the grand potential of the mixed quantum-classical system as a variational problem which depends only on the nuclear density matrix, which experiences an external field generated by the electronic and classical one-particle densities. In practice, the grand potential is computed by a series of coupled classical and quantum DFT calculations, together with geometry optimization.

physics.chem-ph↗

arXiv2025

A Molecular Density Functional Theory of aqueous electrolytic solution

We propose a generalisation of molecular density functional theory to describe inhomogeneous solvent mixture, with the objective of modelling electrolytic solutions. Two electrolytic models are presented, both within the HNC approximation. The first one is a two-components mixture representing a primitive-like model of sodium chloride, where the solvent is described as a dielectric continuum. This popular model has the advantage of simplicity, as the ions densities solely depend on spatial coordinates. Additionally, we develop a realistic three-components electrolyte model, in which water solvent is described by a third density field that depends on both spatial and orientational coordinates. The proposed methodology and its tridimensional implementation (3 spatial coordinates and 3 Euler angles) are validated by comparing the solvation properties of a sodium cation with the predictions of integral equation theory solved in 1D (1 intermolecular distance and 5 Euler angles), showing near-perfect agreement. This methodology enables the study of solvation properties of solutes of arbitrary shapes in electrolytic solutions, as demonstrated with the prototypical N-methylacetamide molecule immersed in both electrolytic solution models.

physics.chem-ph↗

arXiv2025

Ions at electrochemical interfaces: from explicit to implicit molecular solvent descriptions

We investigate the interplay between electronic screening inside a metal and screening by a polar molecular solvent, focusing on their impact on the charge induced by an ion and the solvent structure at the interface. To that end, we consider atomistically resolved electrodes within the Thomas-Fermi model of screening and describe the molecular solvent either explicitly via classical molecular dynamics or implicitly using Molecular Density Functional Theory (MDFT). Specifically, we examine the effect of screening by tuning the Thomas-Fermi screening length $l_\text{TF}$, the ion charge by considering Na$^+$ and Cl$^-$ and the solvent nature by studying water and acetonitrile. Consistently with our previous findings without solvent, $l_\text{TF}$ significantly affects the charge distribution inside the metal. However, $l_\text{TF}$ has no significant impact on the interfacial solvent structure, suggesting that its effect on the charge distribution induced inside the metal by the ion is essentially due to how the metal responds to the (same) external charge distribution, including the solvent, even though the coupling between both sides of the interface may play a secondary role. Furthermore, MDFT accurately reproduces fine details of the interfacial solvent structure around the ion at a fraction of the computational cost of MD simulations. These results highlight the relevance of MDFT as a powerful tool to model electrochemical systems at the molecular level.

physics.chem-ph↗

arXiv2024

Coupling Molecular Density Functional Theory with Converged Selected Configuration Interaction Methods to Study Excited states in Aqueous Solution

This paper presents the first implementation of a coupling between advanced wave function theories and molecular density functional theory (MDFT). This method enables the modeling of solvent effect into quantum mechanical (QM) calculations by incorporating an electrostatic potential generated by solvent charges into the electronic Hamiltonian. Solvent charges are deduced from the spatially and angularly dependent solvent particle density. Such density is obtained through the minimization of the functional associated to the molecular mechanics (MM) Hamiltonian describing the interaction between the fluid particles. The introduced QM/MDFT framework belongs to QM/MM family of methods but its originality lies in the use of MDFT as the MM solver, offering two main advantages. Firstly, its functional formulation makes it competitive with respect to sampling-based molecular mechanics. Secondly, it preserves a molecular-level description lost in macroscopic continuum approaches. Excited states properties of water and formaldehyde molecules solvated into water have been computed at the selected configuration interaction (SCI) level. Excitation energies and dipole moment have been compared with experimental data and previous theoretical work. A key finding is that using the Hartree-Fock method to describe the solute allows for predicting the solvent charge around the ground-state with sufficient precision for the subsequent SCI calculations of excited-states. This significantly reduces the computational cost of the described procedure, paving the way for the study of more complex molecules.

physics.chem-ph↗

arXiv2023

Dielectric response of confined water films: Insights from classical DFT

We re-examine the problem of the dielectric response of highly polar liquids such as water in confinement between two walls using a simple two-variable density functional theory involving number and polarisation densities. In the longitudinal polarisation case where a perturbing field is applied perpendicularly to the walls, we show that the notion of local dielectric constant, although ill-defined at a microscopic level, makes sense when a coarse-graining over the typical size of a particle is introduced. The approach makes it possible to study the effective dielectric response of thin liquid films of various thicknesses in connection to the recent experiments of [Fumagalli et al. , Science, 2018, 360, 1339-1342], and to discuss the notion interfacial dielectric constant. We argue that the observed properties as function of slab dimension, in particular the very low dielectric constants of the order of 2-3 measured for thin slabs of 1 nm thickness do not highlight any special property of water but can be recovered for a generic polar solvent having similar particle size and the same high dielectric constant. Regarding the transverse polarisation case where the perturbing field is parallel to the walls, the associated effective dielectric constant as a function of the slab dimension reaches bulk-like values at much shorter widths than in the longitudinal case.

cond-mat.stat-mech↗

arXiv2021

Multi-scale simulation of the adsorption of lithium ion on graphite surface: from Quantum Monte Carlo to Molecular Density Functional Theory

The structure of the double-layer formed at the surface of carbon electrodes is governed by the interactions between the electrode and the electrolyte species. However, carbon is notoriously difficult to simulate accurately, even with well-established methods such as electronic Density Functional Theory and Molecular Dynamics. Here we focus on the important case of a lithium ion in contact with the surface of graphite, and we perform a series of reference Quantum Monte Carlo calculations that allow us to benchmark various electronic Density Functional Theory functionals. We then fit an accurate carbon--lithium pair potential, which is used in molecular Density Functional Theory calculations to determine the free energy of the adsorption of the ion on the surface in the presence of water. The adsorption profile in solution differs markedly from the gas phase results, which emphasize the role of the solvent on the properties of the double-layer.

cond-mat.mtrl-sci↗

arXiv2021

Assessing the correctness of pressure correction to solvation theories in the study of electron transfer reactions

Liquid states theories have emerged as a numerically efficient alternative to costly molecular dynamics simulations of electron transfer reactions in solution. In a recent paper [Chem. Sci., 2019, 10, 2130], we introduced the framework to compute energy gap, free energy profile and reorganization free energy using molecular density functional theory. However, this technique, as other molecular liquid state theories, overestimates the bulk pressure of the fluids. Because of the too high pressure, the predicted free energy is dramatically exaggerated. Several attempts were made to fix this issue, either based on simple a posteriori correction or by improving the description of the liquid introducing bridge terms. By studying two model half reactions in water, Cl -> Cl+ and Cl -> Cl-, we assess the correctness of these two types of corrections to study electron transfer reactions. We found that a posteriori corrections, because they violate the functional principle, lead to an inconsistency in the definition of the reorganization free energy and should not be used to study electron transfer reactions. The bridge approach, because it is theoretically well grounded, is perfectly suitable for this type of systems.

physics.chem-ph↗

arXiv2020

Tackling solvent effect by coupling electronic and molecular Density Functional Theory

Solvation effect might have a tremendous influence on chemical reactions. However, precise quantum chemistry calculations are most often done either in vacuum neglecting the role of the solvent or using continuum solvent model ignoring its molecular nature. We propose a new method coupling a quantum description of the solute using electronic density functional theory with a classical grand-canonical treatment of the solvent using molecular density functional theory. Unlike previous work, both densities are minimized self consistently, accounting for mutual polarization of the molecular solvent and the solute. The electrostatic interaction is accounted using the full electron density of the solute rather than fitted point charges. The introduced methodology represents a good compromise between the two main strategies to tackle solvation effect in quantum calculation. It is computationally more effective than a direct quantum-mechanics/molecular mechanics coupling, requiring the exploration of many solvent configurations. Compared to continuum methods it retains the full molecular-level description of the solvent. We validate this new framework onto two usual benchmark systems: a water solvated in water and the symmetrical nucleophilic substitution between chloromethane and chloride in water. The prediction for the free energy profiles are not yet fully quantitative compared to experimental data but the most important features are qualitatively recovered. The method provides a detailed molecular picture of the evolution of the solvent structure along the reaction pathway.

physics.chem-ph↗

arXiv2019

A first-principles investigation of the structural and electrochemical properties of biredox ionic species in acetonitrile

Biredox ionic liquids are a new class of functionalized electrolytes that may play an important role in future capacitive energy storage devices. By allowing additional storage of electrons inside the liquids, they can improve device performance significantly. However current devices employ nanoporous carbons in which the diffusion of the liquid and the adsorption of the ions could be affected by the occurrence of electron-transfer reactions. It is therefore necessary to understand better the thermodynamics and the kinetics of such reactions in biredox ionic liquids. Here we perform ab initio molecular dynamics simulations of both the oxidized and reduced species of several redox-active ionic molecules (used in biredox ionic liquids) dissolved in acetonitrile solvent and compare them with the bare redox molecules. We show that in all the cases, it is necessary to introduce a two Gaussian state model to calculate the reaction free energies accurately. These reaction free energies are only slightly affected by the presence of the IL group on the molecule. We characterize the structure of the solvation shell around the redox active part of the molecules and show that in the case of TEMPO-based molecules strong reorientation effects occur during the oxidation reaction.

cond-mat.mtrl-sci↗

arXiv2019

Simulating electrochemical systems by combining the finite field method with a constant potential electrode

A better understanding of interfacial mechanisms is needed to improve the performances of electrochemical devices. Yet, simulating an electrode surface at fixed electrolyte composition remains a challenge. Here we apply a finite electric field to a single electrode held at constant potential and in contact with an aqueous ionic solution, using classical molecular dynamics. The polarization yields two electrochemical interfaces on opposite sides of the same metal slab. While the net charge on one electrode surface is the opposite of the net charge on the other, maintaining overall charge neutrality of the metal. The electrode surface charges fluctuations are compensated by the adsorption of ions from the electrolyte, forming a pair of electric double layers with aligned dipoles. This opens the way towards the efficient simulation of electrochemical interfaces using any flavor of molecular dynamics, from classical to first principles-based methods.

cond-mat.mtrl-sci↗

arXiv2019

Study of a water-graphene capacitor with molecular density functional theory

Most of the performances of electrochemical devices are governed by molecular processes taking place at the solution-electrode interfaces and molecular simulation are the main way to study these processes. Aqueous electrochemical systems have often been studied using classical DFT but with too crude approximations to consider the system description to be realistic. We study the interface between graphene electrodes and liquid water at different applied voltage using molecular DFT, improving the state of the art by the following key points: 1) electrodes have a realistic atomic resolution, 2) classical DFT calculations are carried out at fixed imposed potential difference and 3) water is described by a molecular model. This allows to reveal the structural modification of water adsorbed at the graphene interface and the evolution of water dielectric permittivity when a voltage is applied. The computed capacitance of this device is in agreement with molecular dynamics simulations. This demonstrates the relevance of molecular DFT to study electrochemical systems at the molecular level.

physics.chem-ph↗

arXiv2018

A Molecular Density Functional Theory Approach to Electron Transfer Reactions

Beyond the dielectric continuum description initiated by Marcus theory, the nowadays standard theoretical approach to study electron transfer (ET) reactions in solution or at interfaces is to use classical force field or ab initio Molecular Dynamics simulations. We propose here an alternative method based on liquid-state theory, namely molecular density functional theory, which is numerically much more efficient than simulations while still retaining the molecular nature of the solvent. We begin by reformulating molecular ET theory in a density functional language and show how to compute the various observables characterizing ET reactions from an ensemble of density functional minimizations. In particular, we define in that formulation the relevant order parameter of the reaction, the so-called vertical energy gap, and determine the Marcus free energy curves of both reactant and product states along that coordinate. Important thermodynamic quantities such as the reaction free energy and the reorganization free energies follow. We assess the validity of the method by studying the model Cl$^0\rightarrow$ Cl$^+$ and Cl$^0\rightarrow$ Cl$^-$ ET reactions in bulk water for which molecular dynamics results are available. The anionic case is found to violate the standard Marcus theory. Finally, we take advantage of the computational efficiency of the method to study the influence of confinement on the ET, by investigating the evolution of the reorganization free energy of the Cl$^0\rightarrow$ Cl$^+$ reaction when the atom approaches an atomistically resolved wall.

physics.chem-ph↗

arXiv2017

Confinement Effects on an Electron Transfer Reaction in Nanoporous Carbon Electrodes

Nanoconfinement generally leads to drastic effect on the physical and chemical properties of ionic liquids. Here we investigate how the electrochemical reactivity in such media may be impacted inside nanoporous carbon electrodes. To this end, we study a simple electron transfer reaction using molecular dynamics simulations. The electrodes are held at constant electric potential by allowing the atomic charges on the carbon atoms to fluctuate. We show that the $\mathrm{Fe^{3+}}/\mathrm{Fe^{2+}}$ couple dissolved in an ionic liquid exhibits a deviation with respect to Marcus theory. This behavior is rationalized by the stabilization of a solvation state of the Fe$^{3+}$ cation in the disordered nanoporous electrode that is not observed in the bulk. The simulation results are fitted with a recently proposed two solvation state model, which allows us to estimate the effect of such a deviation on the kinetics of electron transfer inside nanoporous electrodes.

cond-mat.mtrl-sci↗

arXiv2017

Semistochastic Heat-bath Configuration Interaction method: selected configuration interaction with semistochastic perturbation theory

We extend the recently proposed heat-bath configuration interaction (HCI) method [Holmes, Tubman, Umrigar, J. Chem. Theory Comput. 12, 3674 (2016)], by introducing a semistochastic algorithm for performing multireference Epstein-Nesbet perturbation theory, in order to completely eliminate the severe memory bottleneck of the original method. The proposed algorithm has several attractive features. First, there is no sign problem that plagues several quantum Monte Carlo methods. Second, instead of using Metropolis-Hastings sampling, we use the Alias method to directly sample determinants from the reference wavefunction, thus avoiding correlations between consecutive samples. Third, in addition to removing the memory bottleneck, semistochastic HCI (SHCI) is faster than the deterministic variant for many systems if a stochastic error of 0.1 mHa is acceptable. Fourth, within the SHCI algorithm one can trade memory for a modest increase in computer time. Fifth, the perturbative calculation is embarrassingly parallel. The SHCI algorithm extends the range of applicability of the original algorithm, allowing us to calculate the correlation energy of very large active spaces. We demonstrate this by performing calculations on several first row dimers including F2 with an active space of (14e, 108o), Mn-Salen cluster with an active space of (28e, 22o), and Cr2 dimer with up to a quadruple-zeta basis set with an active space of (12e, 190o). For these systems we were able to obtain better than 1 mHa accuracy with a wall time of merely 55 seconds, 37 seconds, and 56 minutes on 1, 1, and 4 nodes, respectively.

physics.chem-ph↗

arXiv2016

Stochastic multi-reference perturbation theory with application to linearized coupled cluster method

In this article we report a stochastic evaluation of the recently proposed LCC multireference perturbation theory [Sharma S., and Alavi A., J. Chem. Phys. 143, 102815, (2015)]. In this method both the zeroth order and first order wavefunctions are sampled stochastically by propagating simultaneously two populations of signed walkers. The sampling of the zeroth order wavefunction follows a set of stochastic processes identical to the one used in the FCIQMC method. To sample the first order wavefunction, the usual FCIQMC algorithm is augmented with a source term that spawns walkers in the sampled first order wavefunction from the zeroth order wavefunction. The second order energy is also computed stochastically but requires no additional overhead outside of the added cost of sampling the first order wavefunction. This fully stochastic method opens up the possibility of simultaneously treating large active spaces to account for static correlation and recovering the dynamical correlation using perturbation theory. This method is used to study a few benchmark systems including the carbon dimer and aromatic molecules. We have computed the singlet-triplet gaps of benzene and m-xylylene. For m-xylylene, which has proved difficult for standard CASSCF+PT, we find the singlet-triplet gap to be in good agreement with the experimental values.

physics.chem-ph↗