Research Project
Full description 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 between the high spin (HS) and low spin (LS) states has been plagued with difficulties. This is a study of a collection of Fe(II) and Fe(III) materials, where the change in enthalpy has been measured. The best performing hybrid functional, TPSSh, achieves a mean absolute error compared to experiment of 11kJ/mol 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 SCO 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 for an effective potential of 1.6 eV. However, the MAE for both TPSSh and DFT+U are dominated by a single material - 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.