Comparison of the Becke-Lee-Yang-Parr and Becke-Perdew-Wang exchange-correlation functionals for geometries of cyclopentadienyl-transition metal complexes

Complete all-electron density functional geometry optimizations have been performed on six "half-sandwich" cyclopentadienyl-transition metal complexes using both the Becke-Lee-Yang-Parr (BLYP) and Becke-Perdew-Wang functionals (BPW91). The two functionals agree very well with each other for bond distances not involving the metal. In a double-ζ plus polarization quality basis, both overestimate these distances by amounts similar to those observed in small molecules with equivalent basis sets. For the single first series complex examined, BPW91 gives comparable agreement for all the bonds, but for complexes of the second transition series, BPW91 consistently overestimates the metal-Cp C distances by 0.06-0.07 Å. Metal f functions shorten these slightly. After estimated relativistic contractions are applied, the BPW91 metal-Cp C overestimates for the second series complexes would be reduced to levels comparable to the first series or small molecules without metals. However, BLYP consistently gives significantly longer values than BPW91 for all distances involving the metal in both the first and second series complexes, so that even after applying all the previous corrections, overestimates ≥0.07 Å relative to experiment would persist for the metal-Cp C bonds. Consistent disagreements of this magnitude between the two functionals for normal chemical bonds appear to be unprecedented in the literature.