CASPT2 molecular geometries of Fe(II) spin-crossover complexes

Abstract

Using fully internally contracted (FIC)-CASPT2 analytical gradients, geometry optimizations of spin-crossover complexes are reported. This approach is tested on a series of Fe(II) complexes with different sizes, ranging from 13 to 61 atoms. A combination of active space and basis set choices are employed to investigate their role in determining reliable molecular geometries. The reported strategy demonstrates that a wave function-based level of theory can be used to optimize the geometries of metal complexes in reasonable times and enables one to treat the molecular geometry and electronic structure of the complexes using the same level of theory. For a series of smaller Fe(II) SCO complexes, strong field ligands in the LS state result in geometries with the largest differences between DFT and CASPT2; however, good agreement overall is observed between DFT and CASPT2. For the larger complexes, moderate sized basis sets yield geometries that compare well with DFT and available experimental data. We recommend using the (10e,12o) active space since convergence to a minimum structure was more efficient than with truncated active spaces (e.g., (6e,5o)) despite having similar Fe–ligand bond distances.

Importance of Dispersion in the Molecular Geometries of Mn(III) Spin Crossover Complexes

“…The authors have performed full CASPT2 geometry optimizations for a series of Fe(II) spin crossover complexes of different size, where DFT is known to provide reliable geometries. 43 The CASPT2 geometries show accuracy no less than those produced by DFT. The feasibility of such computations on larger complexes as well as the effect of active space and basis set choice were also demonstrated.…”