Intra- and Interatomic Spin Interactions by the Density Functional Theory plus U Approach: A Critical Assessment

Accurate evaluation of the total energy difference between different spin states in molecular magnetic systems is currently a great challenge in theoretical chemistry. In this work we assess the performance of the density functional theory plus the Hubbard U (DFT+U) approach for the first-principles description of the high spin-low spin (HS-LS) Splitting and the exchange coupling constant, corresponding to the intra- and interatomic spin interactions, respectively. The former is investigated using a set of mononuclear ion complexes with different HS-LS splitting, including seven spin-crossover (SCO) compounds, while the latter is investigated in a series of binuclear copper complexes covering both ferromagnetic and antiferromagnetic interactions. We find that the DFT+U approach can reproduce experimental data as accurately as the hybrid functionals approach but with much lower computational efforts. We further analyze the effect of U in. terms of spin density on magnetic centers, and we find that the main effect of the U correction can be attributed to the enhanced localization of magnetic orbitals. Even taking the uncertainty related to the determination of U into account, we think the DFT+U approach is an efficient and predictive first-principles method for the SCO phenomenon and interatomic magnetic interactions.