Équipe Modélisation Quantique

L’équipe MQ s’intéresse aux simulations des matériaux par les méthodes de la fonctionnelle de la densité. Notre contre d’intérêt et l’élaboration de nouvelles méthodologies pour traiter les forces van der Waals (de dispersion de London) aussi bien dans les systèmes périodique que dans les complexes intermoléculaires. Des nouvelles approches ont été développées dans un cadre de sépartion de portée des interactions interélectroniques en faisant appel aux techniques basées sur la RPA. La structure électronique des matériaux et les états excités sont étudiés par des méthodes Kohn-Sham et post-Kohn-Sam (GW, BSE) ainsi qu’avec l’approche TDDFT.

Highlights

  • Nouvelle méthode pour l’énergie de corrélation par RPA

Random-phase approximation correlation energies from Lanczos chains and an optimal basis set: Theory and applications to the benzene dimer D. Rocca, J. Chem. Phys., 140 (2014) 18A501.

benzene-dimer

A new ab initio approach is introduced to compute the correlation energy within the adiabatic connection fluctuation dissipation theorem in the random phase approximation. First, an optimally small basis set to represent the response functions is obtained by diagonalizing an approximate dielectric matrix containing the kinetic energy contribution only. Then, the Lanczos algorithm is used to compute the full dynamical dielectric matrix and the correlation energy. The convergence issues with respect to the number of empty states or the dimension of the basis set are avoided and the dynamical effects are easily kept into account. To demonstrate the accuracy and efficiency of this approach the binding curves for three different configurations of the benzene dimer are computed: T-shaped, sandwich, and slipped parallel.

 

  • Dispersion correction in DFT

Improved Density Dependent Correction for the Description of London Dispersion Forces T. Bucko, S. Lebègue, J. Hafner, J.G. Ángyán, J. Chem. Theory Comput., 9 (2013) 4293.

ct-2013-00694h_0003

The Tkatchenko-Scheffler method for calculating dispersion correction to standard density-functional theory, which uses fixed neutral atoms as a reference to estimate the effective volumes of atoms-in-molecule and to calibrate their polarizabilities and dispersion coefficients, fails to describe the structure and the energetics of ionic solids. Here, we propose a more appropriate partitioning, based on the iterative Hirshfeld scheme, where the fractionally charged atomic reference state is determined self-consistently. We show that our new method extends the applicability of the original method in particular to study ionic systems and adsorption phenomena on surfaces of ionic solids.

 

  • Structure prediction

Two-Dimensional Materials from Data Filtering and Ab Initio Calculations S. Lebègue, T. Björkman, M. Klintenberg, R. M. Nieminen, and O. Eriksson, Phys. Rev. X, 3 (2013) 031002.

PhysRevX.3.031002

Materials with reduced dimensionality are promising for applications such as electronic devices, solid-state lubricants, and sensors. Graphene—a monolayer of carbon atoms packed into a planar honeycomb lattice—is currently attracting the most attention because it possesses an unusual electronic structure, is very stable mechanically, and has a high thermal conductivity. Graphene, however, lacks a finite band gap, which is a necessary condition for using it in many electronic devices. Recent work suggests that the two-dimensional systems boron nitride and the transition-metal dichalcogenides are promising graphene alternatives, as they are semiconductors with finite band gaps. But, the number of systems explored so far has been limited because the synthesis and characterization of new materials are difficult and time consuming.
In this paper, we report a two-step computational approach to discovering new two-dimensional materials and studying their properties that is both cost effective and efficient. First, we define a set of geometrical characteristics, such as packing density and lattice constant asymmetry, that are associated with certain van der Waals bonded, layered structures (like graphite, the parent compound of graphene). We then screen the Inorganic Crystal Structure Database for compounds that match these criteria. This procedure has identified 92 compounds that could be easily exfoliated into two-dimensional sheets. Of these, we propose 40 for the first time as parent compounds for two-dimensional materials. Second, we use density-functional theory to calculate the electronic structure (density of states and band dispersion) of the two-dimensional building blocks of these compounds. These calculations predict whether the two-dimensional material is metallic or insulating, as well as if it undergoes magnetic ordering at low temperatures.
Our work suggests that the number of two-dimensional materials, and the variety of electronic structures these materials can form, is quite large. We expect this work will pave the way for further experimental and theoretical study in the field of functionalized materials science.

 

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