This research project develops a rigorous and comprehensive mathematical understanding of models in Quantum Chemistry in the presence of an external magnetic field. The programme addresses the existence and uniqueness of a ground state and excited states, for various fields in (i) Hartree-Fock type models, and (ii) Thomas-Fermi type models. Moreover, the objective is to identity, explore and develop relevant links between the various Variational Methods, with a view to providing a more integrated approach to their use in Nonlinear Partial Differential Equations and initiating new areas of application within Quantum Chemistry, with possible use in numerical studies.