Quantitative computational thermochemistry of transition metal species

The correlation consistent Composite Approach (ccCA), which has been shown to achieve chemical accuracy (+/- l kcal mol(-1)) for a large benchmark set of main group and s-block metal compounds, is used to compute enthalpies of formation for a set of 17 3d transition metal species. The training set includes a variety of metals, ligands, and bonding types. Using the correlation consistent basis sets for the 3d transition metals, we find that gas-phase enthalpies of formation can be efficiently calculated for inorganic and organometallic molecules with ccCA. However, until the reliability of gas-phase transition metal thermochemistry is improved, both experimentally and theoretically, a large experimental training set where uncertainties are near +/- 1 kcal mol(-1) (akin to commonly used main group benchmarking sets) remains an ambitious goal. For now, an average deviation of +/- 3 kcal mol(-1) appears to be the initial goal of "chemical accuracy" for ab initio transition metal model chemistries. The ccCA is also compared to a more robust but relatively expensive composite approach primarily utilizing large basis set coupled cluster computations. For a smaller training set of eight molecules, ccCA has a mean absolute deviation (MAD) of 3.4 kcal mol(-1) versus the large basis set coupled-cluster-based model chemistry, which has a MAD of 3.1 kcal mol(-1). However, the agreement for transition metal complexcs is more system dependent than observed in previous benchmark studies of composite methods and main group compounds.