Siig Oliver S, Kepp Kasper P
DTU Chemistry , Technical University of Denmark , Building 206 , 2800 Kgs. Lyngby , Denmark.
J Phys Chem A. 2018 Apr 26;122(16):4208-4217. doi: 10.1021/acs.jpca.8b02027. Epub 2018 Apr 13.
Spin crossover (SCO) plays a major role in biochemistry, catalysis, materials, and emerging technologies such as molecular electronics and sensors, and thus accurate prediction and design of SCO systems is of high priority. However, the main tool for this purpose, density functional theory (DFT), is very sensitive to applied methodology. The most abundant SCO systems are Fe(II) and Fe(III) systems. Even with average good agreement, a functional may be significantly more accurate for Fe(II) or Fe(III) systems, preventing balanced study of SCO candidates of both types. The present work investigates DFT's performance for well-known Fe(II) and Fe(III) SCO complexes, using various design types and customized versions of GGA, hybrid, meta-GGA, meta-hybrid, double-hybrid, and long-range-corrected hybrid functionals. We explore the limits of DFT performance and identify proficient Fe(II)-Fe(III)-balanced functionals. We identify and quantify remarkable differences in the DFT description of Fe(II) and Fe(III) systems. Most functionals become more accurate once Hartree-Fock exchange is adjusted to 10-17%, regardless of the type of functionals involved. However, this typically introduces a clear Fe(II)-Fe(III) bias. The most accurate functionals measured by mean absolute errors <10 kJ/mol are CAMB3LYP-17, B3LYP*, and B97-15 with 15-17% Hartree-Fock exchange, closely followed by CAMB3LYP and CAMB3LYP-15, OPBE, rPBE-10, and B3P86-15. While GGA functionals display a small Fe(II)-Fe(III) bias, they are generally inaccurate, except the O exchange functional. Hybrid functionals (including B2PLYP double hybrids and meta hybrids) tend to favor HS too much in Fe(II) vs Fe(III), which is important in many studies where the oxidation state of iron can vary, e.g. rational SCO design and studies of catalytic processes involving iron. The only functional with a combined bias <5 kJ/mol and a decent MAE (15 kJ/mol) is our customized PBE0-12 functional. Alternatively one has to sacrifice Fe(II)-Fe(III) balance to use the best functionals for each group separately. We also investigated the precision (measured as the standard deviation of errors) and show that the target accuracy for iron SCO is 10 kJ/mol for accuracy and 5 kJ/mol for precision, and DFT is probably not going to break this limit in the near future. Importantly, all four types of functional behavior (accurate/precise, accurate/imprecise, inaccurate/precise, inaccurate/imprecise) are observed. More generally, our work illustrates the importance not only of overall accuracy but also of balanced accuracy for systems likely to occur in context.
自旋交叉（SCO）在生物化学、催化、材料以及诸如分子电子学和传感器等新兴技术中发挥着重要作用，因此对SCO系统进行准确预测和设计具有高度优先性。然而，用于此目的的主要工具——密度泛函理论（DFT），对所应用的方法非常敏感。最丰富的SCO系统是Fe（II）和Fe（III）系统。即使总体上有较好的一致性，一种泛函对于Fe（II）或Fe（III）系统可能会显著更准确，这阻碍了对这两种类型的SCO候选物进行平衡研究。本工作使用各种设计类型以及GGA、杂化、元GGA、元杂化、双杂化和长程校正杂化泛函的定制版本，研究了DFT对著名的Fe（II）和Fe（III）SCO配合物的性能。我们探索了DFT性能的极限，并确定了熟练的Fe（II）-Fe（III）平衡泛函。我们识别并量化了DFT对Fe（II）和Fe（III）系统描述中的显著差异。一旦将Hartree-Fock交换调整到10 - 17%，大多数泛函都会变得更准确，而与所涉及的泛函类型无关。然而，这通常会引入明显的Fe（II）-Fe（III）偏差。通过平均绝对误差<10 kJ/mol衡量的最准确泛函是具有15 - 17% Hartree-Fock交换的CAMB3LYP - 17、B3LYP*和B97 - 15，紧随其后的是CAMB3LYP和CAMB3LYP - 15、OPBE、rPBE - 10以及B3P86 - 15。虽然GGA泛函显示出较小的Fe（II）-Fe（III）偏差，但它们通常不准确，除了O交换泛函。杂化泛函（包括B2PLYP双杂化和元杂化）在Fe（II）与Fe（III）中往往过于倾向高自旋态（HS），这在许多铁的氧化态可能变化的研究中很重要，例如合理的SCO设计以及涉及铁的催化过程研究。唯一一种综合偏差<5 kJ/mol且平均绝对误差良好（15 kJ/mol）的泛函是我们定制的PBE0 - 12泛函。或者，为了分别对每组使用最佳泛函，不得不牺牲Fe（II）-Fe（III）平衡。我们还研究了精度（以误差的标准偏差衡量），并表明铁SCO的目标精度为10 kJ/mol，精度为5 kJ/mol，并且DFT在不久的将来可能无法突破这个极限。重要的是，观察到了所有四种泛函行为类型（准确/精确、准确/不精确、不准确/精确、不准确/不精确）。更一般地说，我们的工作不仅说明了整体准确性的重要性，还说明了对于可能在实际中出现的系统而言平衡准确性的重要性。
