YUAN Xiang: Molecular properties in the linear response regime and beyond.
PhD thesis: The study of molecular properties allows us to characterize the electronic structure of atoms, molecules and materials, and to provide an understanding of the parameters controlling appealing orinteresting features (absorption or emission at certain wavelengths etc.). Such properties can be defined in the context of the (electronic) energy of a system with respect to (external or internalperturbations) , and in recent years there is remarkable progress in their determination inboth the linear (electronic excitation spectra, polarizabilities, NMR shieldings etc) and non-linear (hyperpolarizabilities, optical Kerr effect, birefringences, etc.) using simulations based on the equation of the Schrödinger equation for the electrons, with analytic derivatives techniques and accuratemany-body approaches based on clustered formalism to describe electron correlation. Thereare cases, however, for which special relativity must be taken into account in the context of the periodic table of the world, and the effects of the X-ray absoprtion and photoelectron spectra. thus calling for the solution of the Dirac equation combined with the clustered approaches [2,3]. In the relativistic case, however, this time can only be compared to the correlation based on density functional theory (DFT) . The goal of this thesis is therefore to develop the relativistic equation of motion coupled cluster approach  for the determination of linear and non-linear molecular properties, with a particular focus on the properties of two-photonabsorption cross-sections (a non-linear This invention is related to the use of magnetic resonance spectra, which can be used in studying species with complicated electronic structures, with often weak absorption signals such as those containing actinides or other metal centers .  ASP Gomes, CR Jacob, Annu. Rep. Dry. C (Phys Chem). 108, 222, 2012  Y Bouchafra, A Shee, Real F, V Vallet, ASP Gomes, Phys. Rev. Lett. 121, 266001 (2018)  ASP Gomes, L Visscher, Bolvin H, T Saue, Knecht S, Fleig T, Eliav E, Chem J. J. Phys., 149, 174113 (2010)  P Tecmer, ASP Gomes, U Ekstrom, L Visscher, Phys. Chem. Chem. Phys., 13, 6249 (2011)  to Shee, T Saue, L Visscher, ASP Gomes, J. Chem. Phys., 149, 174113 (2018)  ASP Gomes, CR Jacob, Real F, L Visscher, V Vallet, Phys. Chem. Chem. Phys., 15, 15153 (2013)