The recovery of quantitatively accurate electron correlation energies can be difficult for many molecules of chemical interest. We are developing a hierarchy of accurate composite theoretical models to overcome these limitations. By combining calculations with high correlation/small basis sets with calculations with low correlation/large basis sets, one can approximate the results of more accurate calculations. We are developing a variety of techniques including novel additive or multiplicative scaling methods to derive results with chemical accuracy (±2 kcal/mol).
Transition metals pose significant difficulties due to the high degree of dynamic electron correlation and the existence of multiple low-lying electronic states. We are actively working toward developing similarly accurate composite methods to treat transition metal containing systems. Success in this area will provide tools for a broad range of problems of which transition metal reactivity is the focus.
