First-Principles Calculation of Transition Metal Hyperfine Coupling Constants with the Strongly Constrained and Appropriately Normed (SCAN) Density Functional and its Hybrid Variants

Dimitrios A Pantazis

doi:10.3390/magnetochemistry5040069

Abstract

Density functional theory (DFT) is used extensively for the first-principles calculation of hyperfine coupling constants in both main-group and transition metal systems. As with many other properties, the performance of DFT for hyperfine coupling constants is of variable quality, particularly for transition metal complexes, because it strongly depends on the nature of the chemical system and the type of approximation to the exchange-correlation functional. Recently, a meta-generalized-gradient approximation (mGGA) functional was proposed that obeys all known exact constraints for such a method, known as the Strongly Constrained and Appropriately Normed (SCAN) functional. In view of its theoretically superior formulation a benchmark set of complexes is used to assess the performance of SCAN for the challenging case of transition metal hyperfine coupling constants. In addition, two global hybrid versions of the functional, SCANh and SCAN0, are described and tested. The values computed with the new functionals are compared with experiment and with those of other DFT approximations. Although the original SCAN and the SCAN-based hybrids may offer improved hyperfine coupling constants for specific systems, no uniform improvement is observed. On the contrary, there are specific cases where the new functionals fail badly due to a flawed description of the underlying electronic structure. Therefore, despite these methodological advances, systematically accurate and system-independent prediction of transition metal hyperfine coupling constants with DFT remains an unmet challenge.

Highlights

Advancements in the theoretical framework for the calculation of electron paramagnetic resonance (EPR) parameters on the basis of Kohn–Sham density functional theory (DFT) [1], combined with the continuous improvement in the quality of approximate exchange–correlation functionals [2,3], have significantly expanded the scope of Density functional theory (DFT) calculations beyond simplistic considerations of potential energy surface features, enabling their tight integration with experimental spectroscopy in modern research [4,5,6]
As with many other properties, the performance of DFT for hyperfine coupling constants is of variable quality, for transition metal complexes, because it strongly depends on the nature of the chemical system and the type of approximation to the exchange-correlation functional
If one is restricted by choice or practical necessity to use DFT instead of rigorous wave function based methods for predicting EPR parameters in transition metal systems, the choice of functional is typically dictated by experience accumulated in previously published studies of related systems or by performing an initial benchmarking study

Summary

Introduction

Advancements in the theoretical framework for the calculation of electron paramagnetic resonance (EPR) parameters on the basis of Kohn–Sham density functional theory (DFT) [1], combined with the continuous improvement in the quality of approximate exchange–correlation functionals [2,3], have significantly expanded the scope of DFT calculations beyond simplistic considerations of potential energy surface features, enabling their tight integration with experimental spectroscopy in modern research [4,5,6]. The underestimation by DFT of core spin polarization can be partly counteracted by admixture of exact exchange in hybrid functionals This is an ad hoc fix that may introduce other problems, for example related to spin contamination, and adversely affect contributions to hyperfine coupling other than the Fermi contact term. Studies that examined the performance of SCAN and certain derivative functionals have already indicated that it performs well for a variety of energy-related properties, usually surpassing previous GGA and mGGA approaches [22,23,24,25,26,27,28,29,30,31] It is unclear whether these advances translate into improved description of the physical mechanisms that give rise to the distinct contributions to hyperfine coupling. A series of methodological considerations regarding the applicability of SCAN are examined

Reference Set of Transition Metal Systems

Performance of SCAN in Comparison With Other Density Functionals

Performance of Hybrid Variants of SCAN

Basis Set Dependence of SCAN Results

Discussion and Conclusions

Computational Details