²³⁷Np Mössbauer Isomer Shifts: A Lesson About the Balance of Static and Dynamic Electron Correlation in Heavy Element Complexes

Abstract

A large set of neptunium compounds with different oxidation states (III to VII) was assembled to study the Mossbauer isomer shift by wave function calculations and better understand covalency in f-elements complexes. The contact density approach was used to calculate the isomer shift using complete active space selfconsistent field (CASSCF) multiconfiguration wave functions, as well as matrix product states [from Density Matrix Renormalization Group (DMRG) algorithms] for large active spaces. Dynamic correlation effects for the isomer shifts were treated via CASPT2 energy derivatives with respect to the nuclear radius. The CASSCF calculations appear to produce different orbital overlocalization errors for low and high Np oxidation states. For compounds with low Np oxidation numbers, the errors can be attributed to the overlocalization of the 5f orbitals. For the compounds with high Np oxidation numbers, the main errors arise from an overlocalization of ligand orbitals and concomitant to weak donation bonding. Attempts to mitigate the overlocalization errors with large active spaces using DMRG were only partially successful, showing that explicit treatment of dynamic correlation is necessary for accurate predictions of Mossbauer isomer shifts. The CASPT2 calculations perform very satisfactorily. For a subset of Np compounds, both static and dynamic correlation effects were substantial. A rational active space selection based on orbital entanglement diagrams proved beneficial for determining the optimal reference wave function.

Theoretical Evaluation of Metal–Ligand Bonding in Neptunium Compounds in Relation to 237Np Mössbauer Spectroscopy

“…The crystal embedding of [Np VI O 6 ] −6 in the reverse neptunyl Na 4 NpO 5 effectively quenches the large intrinsic EFG, resulting in a relatively minor QS compared to the EFG intrinsic to the isolated [Np VI O 6 ] −6 cluster. Calculations of actinide Mossbauer isomer shifts and QS remain challenging, but we hope that the present study and its two predecessors 28,29 show that useful information about actinide covalency and 5f bonding can be obtained by a combination of Mossbauer experiments and first-principles theoretical calculations. The final Mossbauer parameter to be explored in this manner is caused by the magnetic electron− nucleus hyperfine interaction.…”

“…Our second 237 Np Mossbauer study focused on wave function methods and the treatment of static vs dynamic electron correlation. 29 Based on these previous results, most of the outliers seen in Figure 2 can be rationalized by the inadequacy of KS-DFT with available approximate functionals to account for the open-shell electronic structure adopted by these compounds, because of the accompanying pronounced static correlation. An ab initio multiconfigurational approach is necessary to describe the isomer shift for these systems.…”

“…The correlation between the calculated KS-DFT contact densities and the isomer shifts is quite good, overall, recovering the isomer shift ordering of the Np systems, except for some outliers marked in Figure 2. We previously 29 identified the outliers with an inability of KS-DFT to treat the pronounced static correlation in these systems. The present work has focused on the chemical interpretation of the Mossbauer data.…”

“…Using the CASSCF contact densities versus the experimental isomer shift linear relationship from ref 29, the isomer shift for Np II C 5 H 3 (SiH 3 ) 2 is estimated at 34.0 ± 0.2 mm/s, which is similar to the isomer shift estimated from the DFT calculations. It is important to note that CASSCF may underestimate the contact density due to overlocalization of the 5f shell in Np(II) compounds in the absence of dynamic correlation, 29 therefore leading to a somewhat too high isomer shift. The DFT-derived isomer shift of 31 mm/s is likely closer to the truth.…”

“…Thus, a multiconfigurational wave function approach appears to be necessary to describe the QS for these ionic solid outliers. In our most recent study, 29 we demonstrated that a multiconfigurational approach is needed to capture the pronounced static correlation of some Np(VI) ionic solids (e.g., CaNpO 4 ) evidenced in the isomer shift. Therefore, we decided to probe this aspect also in relation to the QS.…”