My main research interest is the electronic structure of solids and intermolecular interactions, with special emphasis on the improvement of density functional theory to take into account dispersion forces. We have developed different approaches to calculats and analyse distributed electrostatic properties (TPEP and OPEP). Interpretation of high resolution X-ray diffraction data obtained in our laboratory offer numerous challenging subjects, like the effect of intermolecular forces on the charge distribution in crystals of organic molecules, topological analysis of experimental and theoretical charge densities, or various aspects of structure and dynamics in microporous materials, neutral and ionic forms of charge transfer complexes and magnetic transitions in transition metal complexes.

• Range-separated density functionals and London dispersion forces
• Charge density analysis
• Geometry optimization of solids using internal coordinates
• Distributed multipoles and polarizabilities (TPEP and OPEP)
• Applications

New methods are implemented mainly in the VASP Vienna ab initio simulation package, and in the Molpro quantum chemistry package. Jointly with C. Chipot (EDAM, Nancy), we have developed the OPEP code to fit distributed multipoles and polarizabilities.

Range-separated hybrid functionals. Inspired by the DFT-CI approach of A. Savin, we have studied the performance of a new kind of hybrid functional, which includes long-range-only exact exchange effects through an attentuated Coulomb operator (e.g. erf(mu r)/r), and treats short-range exchange and correlation are by the local density approximation. While keeping all the favourable properties of the LDA, resulting "range-separated hybrid" functional has further attractive features, like correct asymptotic behaviour of the potential and derivative discontinuity. The single "empirical" parameter, mu characterizing the range of interaction separations, is roughly equal to the inverse of the Thomas-Fermi screening length for valence electrons. Atomizations energies calculated for the G2 set are comparable to the best GGA functionals. Similar tests are in progress for solids.

van der Waals forces in DFT. Present density functionals do not describe correctly the R^-6 asymptotic behaviour of London dispersion (van der Waals) interactions. This is not surprising since these universal attractive forces due to long-range correlation of electrons between weakly- or non-overlapping electron groups. We propose to extend the range-separated hybrid by taking into account long-range correlation effects perturbationally. The RSH+MP2 method seems to be an efficient approach to tackle the dispersion problem. In addition of the rare gas dimers, various applications on difficult cases, like Be₂, benzene dimer, etc are in progress.

These projects are developed in the thesis of Iann Gerber, in collaboration with A. Savin and J. Toulouse (UPMC, Paris).

Several programs are available to perform Bader analysis of molecular and crystal wave functions expanded on Gaussian atomic orbital basis sets. Similar analysis is much more difficult to perform on charge densities resulting from plane wave calculations. As shown by Claudine Katan (Rennes), it is possible in the all-electron PAW formalism of Bloechl. In collaboration with M. Marsman (Vienna), the VASP code has been extended to produce the full charge density on a regular grid and it has been interfaced with the efficient grid-based analysis/integration package Integrity, thus opening the way to perform Bader-analysis on high quality ab initio charge densities routinely.

Geometry optimizations in internal coordinates have been proved to have favourable convergence properties and their use is mandatory for constrained relaxations. A delocalized internal coordinate algorithm has been adapated and programmed in a stand-alone module (Gadget) by T. Bucko (Vienna) . The avantages of this solid optimizer, interfaced with the VASP has been shown; other applications are in progress. This work is closely related to a former collaboration with G. Ferenczy (Budapest) , concerning the modelling of molecular crystals by a semi-empirical QM/MM type approach.

Accurate calculation of electrostatic, induction and dispersion forces in liquids, solids or in biomolecular simulations requires atom-centered multipolar representations of electrostatic and response properties. During the past 10 years, we have developed two complementary approaches.

The TPEP (Topologically Partitioned Electric Properties) method is based on Bader's Atoms in Molecules partition of the charge density and expands static multipole moments as well as static and dynamic polarizabilities in these highly transferable atomic regions . A new updated implementation of this method in the Dalton ab initio package is in progress in collaboration with C. Haettig (Karlsruhe) and the study of optically nonlinear molecular crystals is in progress with H. Reis (Athens).

The OPEP (Optimally Partitioned Electric Properties) approach is essentially a least squares fit method to have an accurate and short expansion of the electrostatic and response properties fitted to the electrostatic and polarization potentials , . The corresponding computer code developed in collaboration with C. Chipot (Nancy), includes a graphical user interface and is freely downloadable from the OPEP homepage.

Electronic structure calculations, icluding geometry optimizations, may be of some help in the better understanding of high resolution X-ray diffraction data obtained in our laboratory on a number of challenging mineral and molecular materials. A few of the on-going collaborations:

Manifestation of intermolecular forces in the reorganization of the charge density (C. Jelsch, B. Guillot, C. Lecomte)

Modelling charge transfer in neutral-ionic complexes of TTF-CA type. (S. Dahaoui, C. Lecomte, C. Katan)

Spin-transition in Fe(btr)₂(NCS)₂](H₂O) (btr=bis-triazole) complexes. (S. Pillet, C. Lecomte)

Electric field induced structural changes in quartz and related materials. (N. Hansen, P. Fertey)

Charge density and intermolecular interactions in microporous materials. (F. Porcher, E. Aubert)

A few further subject in which I am or I have been involved.

• Chemical bond and bond orders
• Solvent effect theory
• Vibrational spectrum of the minerals gibbsite and bayerite (B. Humbert, LCPME)

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