Loïc Halbert PhD defense

Thesis title:

The Equation of Motion Coupled Cluster method for modeling excited states and properties of molecules containing heavy elements

Summary of the thesis:

In this thesis, we seek to obtain certain molecular properties for species containing heavy elements or presenting atmospheric interests. For this, we use techniques to characterize the core electrons, with ionization potentials (IP) or with excitation energies (EE), allowing for example to respectively interpret X-ray Photoelectron Spectroscopy(XPS) and X-ray Absorption Spectroscopy (XAS). We also seek to characterize valence electrons through the polarizability, which is used for example to develop force fields. When we work with heavy elements or with core electrons, we must take relativistic effects into account. We therefore used the Dirac-Coulomb(-Gaunt) Hamiltonian. Furthermore, to compare our results with experiments, we need precise methods. Thus, we will work with the Coupled-Cluster (CC) method, and will use the Equation of Motion Coupled-Cluster (EOM-CC) method to obtain the IPs, EEs and electron affinities (EA). However, these two elements (4-component Hamiltonians and post-Hartree-Fock methods) imply considerable computational costs, requiring the resources of High Performance Computing (HPC) platforms. This thesis presents a study of the Core-Valence Separation (CVS) method, which will allow us to reach the properties of core electrons (IP and EE) with EOM-CC. We provide a detailed investigation of the performance of different Hamiltonians, in particular the exact two-component molecular mean field Hamiltonian. Second, we will focus on the perturbative approximations (Partitioned and Many Body Perturbation Theory 2d order (MBPT(2)) to be applied to the EOM-CC matrix to limit computational costs, including for core processes. Finally, we present the work carried out in Exacorr, a new relativistic coupled cluster implementation for hybrid and massively parallel architectures. We will finish by outlining the formalism and working equations for the Linear Response Coupled-Cluster (LR-CC) method, through which analytical (frequency-dependent) molecular polarizabilities can be obtained.