Ab initio study of magnetic properties from mono and multi determinantal methods
The multidisciplinary interest in magnetic systems stems from the many technological applications in which they are involved, ranging from spintronics (as sensing, memory, or switching devices) to soft matter (as contrast agents for magnetic resonance imaging, spin labels, or mediators for controlled radical polymerization). This work is focused on the theoretical study of their properties. The main characteristics of magnetic systems reside in their intrinsic multireference character that comes from the existence of weakly coupled unpaired electrons. Their theoretical study requires highly correlated treatments and the modelization of their properties usually relies on model Hamiltonians. Ab initio calculations may help to understand the microscopic origin of their macroscopic properties and are often used to evaluate quantitatively the interactions of these model Hamiltonians. The first part of this work concerns the extraction of model Hamiltonian interactions from WFT and DFT calculations. Extractions of Hubbard, Heisenberg, double exchange and Ising Hamiltonians are performed on magnetic spin s=1 systems, mixed valence systems and organic diradicals. The second part deal with the study of iron(II) complexes and their energetic spectra, a fundamental step to determine relaxation process in LIESST compounds (Light Induced Excited Spin State Trapping). Currently, important efforts are made to predict microscopic electronic mechanisms involved in photo-induced spin transitions, but difficulties are still encountered concerning the physics of the excited states. Initially, a study rationalizes the correlation between the structural parameters of these compounds and their LIESST temperature. And finally, a theoretical approach through DFT, TDDFT and WFT calculations is proposed in order to highlight different decay channels.