SCC-DFTB Parameters for Fe–C Interactions

We
present an optimized density-functional tight-binding (DFTB)
parameterization for iron-based complexes based on the popular trans3d set of parameters. The transferability of the original
and optimized parameterizations is assessed using a set of 50 iron
complexes, which include carbonyl, cyanide, polypyridine, and cyclometalated
ligands. DFTB-optimized structures predicted using the trans3d parameters show a good agreement with both experimental crystal
geometries and density functional theory (DFT)-optimized structures
for Fe–N bond lengths. Conversely, Fe–C bond lengths
are systematically overestimated. We improve the accuracy of Fe–C
interactions by truncating the Fe–O repulsive potential and
reparameterizing the Fe–C repulsive potential using a training
set of six isolated iron complexes. The new trans3d*-LANLFeC parameter set can produce accurate Fe–C bond lengths in both
geometry optimizations and molecular dynamics (MD) simulations, without
significantly affecting the accuracy of Fe–N bond lengths.
Moreover, the potential energy curves of Fe–C interactions
are considerably improved. This improved parameterization may open
the door to accurate MD simulations at the DFTB level of theory for
large systems containing iron complexes, such as sensitizer–semiconductor
assemblies in dye-sensitized solar cells, that are not easily accessible
with DFT approaches because of the large number of atoms.