YUAN Xiang: Molecular properties in the linear response regime and beyond.

Thesis summary:

Molecular properties allow us to characterize the electronic structure of atoms, molecules and materials, and from this provide us with an understanding of the physical parameters or phenomena that control the appearance of interesting features (absorption or emission at given wavelengths etc.). Such properties can be defined as derivatives of the (electronic) energy of a system with respect to perturbations (internal or external) [1]. In recent years there have been remarkable advances in the determination of molecular properties in the linear (electronic spectrum, polarizabilities, NMR screening etc.) and non-linear (hyperpolarizabilities, optical Kerr effect, birefringence etc.) response regimes through simulations based on the solution of the Schrödinger equation for electrons by analytical derivative methods and multi-electron wave functions based on the coupled cluster method to describe the electronic correlation of form very precisely. For heavy elements, or properties from internal electrons such as their X-ray absorption or ionization spectra - it is nevertheless necessary to take into account relativity effects through the solution of the Dirac equation in combination with coupled cluster approaches [2, 3]. At this stage, it is only possible to treat the same range of properties (i.e., linear and nonlinear) using non-relativistic and relativistic Hamiltonians, for approximate wavefunctions such as those based on the much less accurate density functional theory (DFT) [4]. Therefore, the objective of this thesis is to develop the equation of motion coupled relativistic cluster method [5] in order to be able to determine linear and non-linear molecular properties, with a particular interest in properties such as two-photon absorption cross sections (a non-linear property) as well as properties arising from both electrical and magnetic perturbations (and which allow us to simulate magnetic circular dichroism spectra), which are very interesting in the characterization of species with a complicated electronic structure and which often present weak dipolar absorption signals, as for molecules containing actinides or metal centers [6]. 

[1] ASP Gomes, CR Jacob, Annu. Rep. Sec. C (Phys. Chem). 108, 222, 2012
[2] Y Bouchafra, A Shee, F Real, V Vallet, ASP Gomes, Phys. Rev. Lett. 121, 266001 (2018)
[3] ASP Gomes, L Visscher, H Bolvin, T Saue, S Knecht, T Fleig, E Eliav, J. Chem. Phys., 149, 174113 (2010)
[4] P Tecmer, ASP Gomes, U Ekstrom, L Visscher, Phys. Chem. Chem. Phys., 13, 6249 (2011)
[5] A Shee, T Saue, L Visscher, ASP Gomes, J. Chem. Phys., 149, 174113 (2018)
[6] ASP Gomes, CR Jacob, F Real, L Visscher, V Vallet, Phys. Chem. Chem. Phys., 15, 15153 (2013)

PhD student: YUAN Xiang - ResearchGate

PhD supervisors: GOMES SEVERO PEREIRA André - VISSCHER Lucas (VU University Amsterdam)