Ohlrich Miriam, Powell Ben J
School of Mathematics and Physics, University of Queensland, Saint Lucia, Queensland 4072, Australia.
J Chem Phys. 2020 Sep 14;153(10):104107. doi: 10.1063/5.0020706.
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, ΔH, 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 ΔH, which determines the low temperature state. Here, we study a collection of Fe(II) and Fe(III) materials, where ΔH 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 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 for U = 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.
自旋交叉材料是双稳态系统，具有作为分子尺度电子开关、致动器、温度计、气压计和显示器的潜在应用。然而，计算高自旋态和低自旋态之间的焓差ΔH一直存在困难。例如，许多常见的密度泛函理论（DFT）方法甚至无法预测ΔH的正确符号，而ΔH决定了低温状态。在这里，我们研究了一组已测量ΔH的Fe(II)和Fe(III)材料，这些材料此前已被用于对密度泛函进行基准测试。对于这组材料，表现最佳的杂化泛函TPSSh与实验相比的平均绝对误差为11 kJ/mol。然而，杂化泛函在固态中的计算量很大；因此，对于研究发生最有趣的自旋交叉现象的晶体材料，局域泛函更可取。我们表明，列支敦士登和杜达列夫的DFT+U方法都比TPSSh稍微准确一些。对于U = 1.6 eV，杜达列夫方法的平均绝对误差为8 kJ/mol。然而，TPSSh和DFT+U的平均绝对误差都由一种单一材料主导，对于这种材料，两种理论方法预测的焓差相似——如果将其从数据集中排除，那么DFT+U就能达到化学精度。因此，DFT+U是计算自旋交叉晶体性质的一个有吸引力的选择，因为它的精度与元杂化泛函相当，但计算成本要低得多。
