We use quantum chemical methods, primarily Density Functional Theory (DFT), to explore the molecular structure, bonding and reactivity of chemical systems, particularly those involving transition metal ions. Transition metal complexes exhibit an enormous variety of structures including classic inorganic coordination compounds, polynuclear transition metal clusters, organometallic complexes and the metal sites in metalloproteins.

The calculation of the electronic structure and properties of these structures however, remains a challenging area of study due to the open-shell nature of many of these systems. Even with the enormous developments in computer hardware, electronic structure calculations on open-shell metal systems using ab initio molecular-orbital methods are still a formidable task, due to the high level of electron correlation present, and really only feasible on the simplest systems. The development of very fast desktop computers and highly efficient computer algorithms in recent years has resulted in DFT becoming an extremely powerful and reliable theoretical technique, ideally suited to the study of transition metal systems, including their structural, magnetic and spectroscopic properties.

Because of the computational expediency of DFT methods, even large structures can be easily handled. Furthermore, the inclusion of electron correlation implicit within DFT ensures that the calculated properties are often in as good or even better agreement with experiment than high-level ab initio methods. More recently, the development of hybrid QM/MM and QM/QM methods has allowed calculations on extremely large systems containing many hundreds or even thousands of atoms, making it now possible to treat biomolecules containing metal ions. Transition metal ions also play a crucial role in a number of important industrial and biological processes, and increasingly DFT is emerging as an indispensable theoretical tool to probe the mechanistic and energetic aspects underlying these processes.