Electronic Structures of Lanthanide Compounds and Aromatic Molecules

Abstract

Lanthanide compounds have wide use in modern technologies. However, the near degeneracy of the 4f orbitals, the spin-orbital coupling, and a large number of electrons in lanthanide compounds make it challenging to study their electronic structures by modern quantum mechanical methods. The photochemical reaction of aromatic molecules is another interesting topic. The first step of photochemical reactions is usually exciting reactants into excited states. Since all the excited states are multiconfigurational which means high static correlation, the study of photochemical reactions is not easy.Most of the modern quantum mechanical methods can be categorized into wave function theory (WFT) methods and density functional theory (DFT) methods. WFT methods are proven to be accurate for multiconfigurational systems while suffering from huge costs for large systems. DFT methods, however, are more economical compared to WFT while suffering from spin contamination when calculating multiconfigurational systems. Multiconfigurational pair-density functional theory (MC-PDFT), extended multi-state PDFT (XMS-PDFT), and compressed-state multi-state PDFT (CMS-PDFT) are newly developed methods that combine the advantage of both WFT and DFT in theory. However, there is not enough validation for PDFT methods in calculating lanthanide compounds and aromatic molecules. The theme of this thesis is to study the electronic structure of complicated lanthanide and aromatic systems and at the same time validate various WFT, DFT, and newly developed PDFT methods in calculating these systems. Chapter 1 introduces the basic concepts of quantum mechanical methods. Chapters 2 and 3 study the lanthanide compounds. The last two chapters, Chapters 4 and 5, analyze the electronic states of aromatic molecules.