Fast, accurate enthalpy differences in spin crossover crystals from DFT plus U

Spin crossover materials are bi-stable systems with potential applications as molecular scale electronic switches, actuators, thermometers, barometers, and displays. However, calculating the enthalpy difference, Delta H, between the high spin and low spin states has been plagued with difficulties. For example, many common density functional theory (DFT) methods fail to even predict the correct sign of Delta H, which determines the low temperature state. Here, we study a collection of Fe(II) and Fe(III) materials, where Delta H has been measured, which has previously been used to benchmark density functionals. The best performing hybrid functional, TPSSh, achieves a mean absolute error compared to experiment of 11 kJ mol(-1) for this set of materials. However, hybrid functionals scale badly in the solid state; therefore, local functionals are preferable for studying crystalline materials, where the most interesting spin crossover phenomena occur. We show that both the Liechtenstein and Dudarev DFT+U methods are a little more accurate than TPSSh. The Dudarev method yields a mean absolute error of 8 kJ mol(-1) for U-eff = 1.6 eV. However, the mean absolute error for both TPSSh and DFT+U is dominated by a single material, for which the two theoretical methods predict similar enthalpy differences-if this is excluded from the set, then DFT+U achieves chemical accuracy. Thus, DFT+U is an attractive option for calculating the properties of spin crossover crystals, as its accuracy is comparable to that of meta-hybrid functionals, but at a much lower computational cost.