Density Functional plus Dynamical Mean-Field Theory of the Spin-Crossover Molecule Fe(phen)$_2$(NCS)$_2$

TL;DR

This paper applies density functional theory combined with dynamical mean-field theory to study the spin-crossover molecule Fe(phen)$_2$(NCS)$_2$, accurately capturing magnetic and energetic properties that are challenging for traditional methods.

Contribution

It demonstrates the effectiveness of DFT+DMFT in accurately modeling spin-crossover molecules and clarifies the differences between DFT+U and SDFT+U approaches.

Findings

Accurate energy difference between high-spin and low-spin states

Reasonably accurate magnetic susceptibility values

Dynamical correlations are essential for low-spin state energetics

Abstract

We study the spin-crossover molecule Fe(phen)$_2$(NCS)$_2$ using density functional theory (DFT) plus dynamical mean-field theory, which allows access to observables not attainable with traditional quantum chemical or electronic structure methods. The temperature dependent magnetic susceptibility, electron addition and removal spectra, and total energies are calculated and compared to experiment. We demonstrate that the proper quantitative energy difference between the high-spin and low-spin state, as well as reasonably accurate values of the magnetic susceptibility can be obtained when using realistic interaction parameters. Comparisons to DFT and DFT+U calculations demonstrate that dynamical correlations are critical to the energetics of the low-spin state. Additionally, we elucidate the differences between DFT+U and spin density functional theory (SDFT) plus U methodologies, demonstrating that DFT+U can recover SDFT+U results for an appropriately chosen on-site exchange interaction.