A general scheme for the construction of exact ground states of correlated electron models will
be discussed. It consists of three steps: The transformation of the Hamiltonian into positive semidefinite
form, the construction of many-particle ground states of this Hamiltonian, and the proof of the
uniqueness of these ground states. This approach works in any dimension, assumes no integrability of
the model and only requires sufficiently many microscopic parameters in the Hamiltonian. Thereby
ground states of diamond Hubbard chains are constructed which exhibit a multitude of properties such
as flat-band ferromagnetism and correlation induced metallic, half-metallic or insulating behavior.
Furthermore, the exact ground state of a 1D periodic Anderson model is shown to explain the unusual
ferromagnetism in the f electron material CeRh3B2. Finally, it is proved that pentagon chain polymers
may be designed to become ferromagnetic or half metallic.