Rodolphe Vuilleumier

Polarizabilities of individual molecules and ions in liquids from first principles

Dipole polarizabilities of individual ionic or molecular species are computed in three different liquid systems: liquid water, molten salts and magmatic melts, the last two belonging to the class of ionic liquids. The method is based on a purely first-principles procedure. The liquid water polarizability tensor is found to be nearly isotropic in the molecular framework. Important environmental effects occur in the two ionic systems when the nature and concentration of the cations are changed. The results of these calculations will be useful in the building of interaction potentials which include polarization effects.

Nuclear velocity perturbation theory for vibrational circular dichroism: An approach based on the exact factorization of the electron-nuclear wave function.

The nuclear velocity perturbation theory (NVPT) for vibrational circular dichroism (VCD) is derived from the exact factorization of the electron-nuclear wave function. This new formalism offers an exact starting point to include correction terms to the Born-Oppenheimer (BO) form of the molecular wave function, similar to the complete-adiabatic approximation. The corrections depend on a small parameter that, in a classical treatment of the nuclei, is identified as the nuclear velocity. Apart from proposing a rigorous basis for the NVPT, we show that the rotational strengths, related to the intensity of the VCD signal, contain a new contribution beyond-BO that can be evaluated with the NVPT and that only arises when the exact factorization approach is employed. Numerical results are presented for chiral and non-chiral systems to test the validity of the approach.

Insight into the Li2CO3–K2CO3 eutectic mixture from classical molecular dynamics: Thermodynamics, structure, and dynamics

We use molecular dynamics simulations to study the thermodynamics, structure, and dynamics of the Li2CO3-K2CO3 (62:38 mol.u2009%) eutectic mixture. We present a new classical non-polarizable force field for this molten salt mixture, optimized using experimental and first principles molecular dynamics simulations data as reference. This simple force field allows efficient molecular simulations of phenomena at long time scales. We use this optimized force field to describe the behavior of the eutectic mixture in the 900-1100 K temperature range, at pressures between 0 and 5 GPa. After studying the equation of state in these thermodynamic conditions, we present molecular insight into the structure and dynamics of the melt. In particular, we present an analysis of the temperature and pressure dependence of the eutectic mixtures self-diffusion coefficients, viscosity, and ionic conductivity.

Reactivity of an Excess Electron with Monovalent Cations in Bulk Water by Mixed Quantum Classical Molecular Dynamics Simulations

A mixed quantum classical molecular dynamics (QCMD) simulation of the silver and sodium cations in presence of an excess electron is reported. The silver cation is shown to be reduced by the hydrated electron and to form a stable, highly polarized, neutral atom. On the contrary, the sodium cation is not reduced and a metastable contact ion pair is observed. The resulting absorption spectra of both species are compared with experiments and shown to be in good agreement. Furthermore, the free energy curve for the charge separation was calculated and rationalized in terms of a thermodynamic cycle. Finally, a direct, reactive, molecular dynamics trajectory provides some useful informations on the reduction mechanism.

TDDFT and Quantum-Classical Dynamics: A Universal Tool Describing the Dynamics of Matter

Time-dependent density functional theory (TDDFT) is currently the most efficient approach allowing to describe electronic dynamics in complex systems, from isolated molecules to the condensed phase. TDDFT has been employed to investigate an extremely wide range of time-dependent phenomena, as spin dynamics in solids, charge and energy transport in nanoscale devices, and photoinduced exciton transfer in molecular aggregates. It is therefore nearly impossible to give a general account of all developments and applications of TDDFT in material science, as well as in physics and chemistry. A large variety of aspects are covered throughout these volumes. In the present chapter, we will limit our presentation to the description of TDDFT developments and applications in the field of quantum molecular dynamics simulations in combination with trajectory-based approaches for the study of nonadiabatic excited-state phenomena. We will present different quantum-classical strategies used to describe the coupled dynamics of electrons and nuclei underlying nonadiabatic processes. In addition, we will give an account of the most recent applications with the aim of illustrating the nature of the problems that can be addressed with the help of these approaches. The potential, as well as the limitations, of the presented methods is discussed, along with possible avenues for future developments in TDDFT and nonadiabatic dynamics.

Maximum probability domains for the analysis of the microscopic structure of liquids

We introduce the concept of maximum probability domains (MPDs), developed in the context of the analysis of electronic densities, in the study of the microscopic spatial structures of liquids. The idea of locating a particle in a three dimensional region, by determining the domain where the probability of finding that, and only that, particle is maximum, gives an interesting characterization of the local structure of the liquid. The optimization procedure, required for the search of the domain of maximum probability, is carried out by the implementation of the level set method. Results for a couple of case studies are presented, to illustrate the structure of liquid water at ambient conditions and upon increasing pressure from the point of view of MPDs and to compare the information encoded in the solvation shells of sodium in water with, once again, that extracted from the MPDs.

CHAPTER 18:Extension of Marcus Rate Theory to Electron Transfer Reactions with Large Solvation Changes

The Marcus theory for charge transfer reaction in solution is reviewed using a molecular statistical mechanics language. This theory is based on the so-called microscopic energy-gap coordinate and relies on a Gaussian solvation picture or, equivalently, on a linear response approximation. It involves two parameters, the reorganization energy and the reaction free energy parameters. However, it may fail when the solvation has a different character in the reactant and product state. For that situation, we describe a complementary theoretical extension, based on a non-Gaussian solvation picture, and we discuss its implications for electron transfer rate constants and rate constant/reaction free energy relationships. Relying on molecular simulation results, we show that such situation arise even for simple half oxido-reduction reactions involving the Cu+/Cu2+ or Ag0 /Ag+ couples or for generic charge transfer reactions involving high polarizability changes. The non-Gaussian theory exhibits the correct non-linear response behavior and reproduces the simulation results quantitatively, whereas a simple one-Gaussian-state Marcus description breaks down.