Density-Functional Based Investigation of Molecular Magnets

Molecules containing transition metal centers provide a complexity that leads to many phenomena of import to chemistry, physics and biology. In particular the metal centers may vary their charge and spin states leading to metal centers that may be viewed classically as closed shell unpolarized states or open-shell spin polarized ions. A large class of molecules containing collections of spin-polarized ions are often referred to as molecular magnets. Even within this somewhat specialized set of molecules, the name molecular magnet is not specific enough as there are two extremes in these systems that behave very differently when exposed to magnetic fields. In this paper we start by discussing the two idealized cases. One idealized class relates to molecules containing metal centers for which the spin-orbit interaction is quite small. The low lying excitations in this class of molecules may be studied from Heisenberg Hamiltonians. Another idealized case corresponds to molecules containing strongly coupled metal centers and non negligible spin-orbit interactions. For this case it is the spin-orbit interaction that is responsible for the lowest energy excitations in the molecule. This class of molecules has received significant experimental attention lately because it exhibits the phenomenon of resonant tunneling of magnetization which is fundamentally interesting and potentially technologically useful. We discuss how density-functional theory may be used to study this class of molecules and provide examples to current systems of interest. The examples show the magnetism in these molecules can be influenced by applied magnetic fields, addition of extra electrons and changes in hydrogen bonding. The discussion includes numerical details in addition to the underlying theoretical framework. While most of this chapter is devoted to our work on this class of anisotropic molecular magnets some discussion of the exchange-coupled idealization is included to contrast the two types of behavior.