Comparison of the accuracy of DFT methods for reactions with relevance to nitrogenase

We have studied the accuracy of 16 different density functional theory methods to reproduce experimental data for bond lengths, angles, vibrational frequencies, as well as enthalpies and entropies for the binding of N2, H2, CO and hydride ions to various transition-metal complexes (with Fe, Ni, Cr, Mo andW) with relation to nitrogenase.We show that generalized gradient approximation functionals give better structure-related parameters, whereas hybrid functionals often give better energies. However, the BLYP and B97D functionals seem to give reasonably accurate results for both types of properties. Geometries and entropies are converged with split-valence basis sets, but energies and vibrational frequencies (and therefore also thermal corrections) in general require a basis set of triple-zeta quality. Dispersion corrections are important to obtain accurate energies (contributing by up to 57 kJ mol-1), as well as structures.