Benchmark study of the Mössbauer isomer shifts of Eu and Np complexes by relativistic DFT calculations for understanding the bonding nature of f-block compounds.

We have performed benchmark investigations into the bonding properties in lanthanide and actinide complexes to quantitatively estimate the covalency of f-block compounds. Three different density functionals including BP86 (pure-GGA), B3LYP (hybrid-GGA) and B2PLYP (double hybrid-GGA) were employed for all-electron self-consistent field calculations compensated by the scalar-relativistic zero-order regular approximation (ZORA) Hamiltonian with a relativistically contracted all-electron basis set. Ten Eu and ten Np complexes were employed as benchmark sets for the calculation of Mössbauer parameters for (151)Eu and (237)Np compounds. As a result of the linear fitting between the calculated electron densities at the nucleus (ρ) and the experimental isomer shifts (δ(exp)), the calculations performed using the all-electron ZORA-B2PLYP level reproduced a change of electron density at the Mössbauer nucleus for both Eu and Np complexes with high correlation coefficients (R(2) > 0.90). Mulliken's population analyses indicated that the BP86 and B3LYP methods overestimated the covalency of both Eu and Np complexes due to the smaller amount of the exact Hartree-Fock exchange admixture included in BP86 and B3PLYP compared to that in the B2PLYP functional. By comparing Mulliken's electronic structure analyses with the experimental isomer shifts, we found that Mulliken's spin population values were good parameters to quantitatively estimate the bonding natures of Eu and Np complexes.