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Conférence Dr. Alessandro Genoni

Vendredi 28 février 2014, 11h00, ICPM-Metz.
"Extremely localized molecular orbitals : past, present and future strategies",
Dr. Alessandro Genoni,
SRSMC, CNRS-Université de Lorraine.

Nowadays, one of the main goals of theoretical chemistry consists in the development of new strategies to study very large systems at a fully quantum mechanical level. In this context, the linear scaling techniques are particularly suitable and, among them, a prominent role is played by the LEGO approaches, which are based on the observation that molecules are generally constituted by functional units (e.g., the traditional functional groups of Organic Chemistry) that keep their main features in different chemical environments. One possible declination of the LEGO methods consists in constructing a database of Molecular Orbitals (MOs) corresponding to all the possible molecular fragments needed to characterize a particular class of molecules. In this way, transferring the MOs from the constructed library to the target structure of a very large system under investigation would allow to almost instantaneously obtain the desired wave function or electron density. Unfortunately, the canonical Hartree-Fock MOs are completely delocalized on the whole molecules on which they are calculated and they cannot be associated with particular functional units. Hence, to overcome this drawback, it is necessary to consider the so-called Extremely Localized Molecular Orbitals (ELMOs), namely orbitals strictly localized on molecular subunits and reliably transferable form molecule to molecule.1, 2 In the first part of the seminar we will show how the ELMOs can be efficiently used to reconstruct the charge distributions of very large macromolecules as proteins. In particular, we will show that, due to the non-orthogonality of the Extremely Localized Molecular Orbitals, this task can be efficiently accomplished only introducing a further approximation, i.e. the “ELMO-Divide & Conquer” approximation3 that exploits the nearsightedness of the electronic structure4 combining the strictly localized nature of the ELMOs with the “Divide & Conquer” philosophy5. After showing that the ELMOs can be really used to determine the charge distributions of large systems, in the second part of the seminar we will describe a strategy that we have recently developed in order to extract Extremely Localized Molecular Orbitals from experimental X-ray diffraction data6, 7. This technique is the first step toward the construction of a new database of X-ray constrained ELMOs that can be considered as an alternative to the already existing theoretical and experimental pseudoatoms libraries, which have been proposed in the framework of the multipole models for the refinement of crystallographic structures and electron distributions of large macromolecules.
1. H. Stoll, G. Wagenblast, H. Preuss, Theoret. Chim. Acta 57, 169 (1980).
2. M. Sironi, A. Genoni, M. Civera, S. Pieraccini, M. Ghitti, Theor. Chem. Acc. 117, 685 (2007).
3. A. Genoni, M. Ghitti, S. Pieraccini, M. Sironi, Chem. Phys. Lett. 415, 256 (2005).
4. E. Prodan, W. Kohn, Proc. Natl. Acad. Sci. USA 102, 11635, 2005.
5. W. Yang, Phys. Rev. Lett. 66, 1438. (1991).
6. A. Genoni, J. Phys. Chem. Lett. 4, 1093 (2013).
7. A. Genoni, J. Chem.Theory Comput. 9, 3004 (2013).