Vela Sergi, Fumanal Maria, Cirera Jordi, Ribas-Arino Jordi
Laboratory for Computational Molecular Design, Institute of Chemical Sciences and Engineering, EPFL, CH-1015 Lausanne, Switzerland.
Laboratoire de Chimie Quantique, UMR 7111, CNRS-Université de Strasbourg, 4 rue Blaise Pascal, F-67000 Strasbourg, France.
Phys Chem Chem Phys. 2020 Mar 4;22(9):4938-4945. doi: 10.1039/d0cp00162g.
The thermal spin crossover (SCO) phenomenon refers to an entropy-driven spin transition in some materials based on d6-d9 transition metal complexes. While its molecular origin is well known, intricate SCO behaviours are increasingly common, in which the spin transition occurs concomitantly to e.g. phase transformations, solvent absorption/desorption, or order-disorder processes. The computational modelling of such cases is challenging, as it requires accurate spin state energies in the solid state. Density Functional Theory (DFT) is the best framework, but most DFT functionals are unable to balance the spin state energies. While a few hybrid functionals perform better, they are still too expensive for solid-state minima searches in moderate-size systems. The best alternative is to dress cheap local (LDA) or semi-local (GGA) DFT functionals with a Hubbard-type correction (DFT+U). However, the parametrization of U is not straightforward due to the lack of reference values, and because ab initio parametrization methods perform poorly. Moreover, SCO complexes undergo notable structural changes upon transition, so intra- and inter-molecular interactions might play an important role in stabilizing either spin state. As a consequence, the U parameter depends strongly on the dispersion correction scheme that is used. In this paper, we parametrize U for nine reported SCO compounds (five based on FeII, 1-5 and four based on FeIII, 6-9) when using the D3 and D3-BJ dispersion corrections. We analyze the impact of the dispersion correction treatments on the SCO energetics, structure, and the unit cell dimensions. The average U values are different for each type of metal ion (FeIIvs. FeIII), and dispersion correction scheme (D3 vs. D3-BJ) but they all show excellent transferability, with mean absolute errors (MAE) below chemical accuracy (i.e. MAE <4 kJ mol-1). This enables a better description of SCO processes and, more generally, of spin state energetics, in materials containing FeII and FeIII ions.
热自旋交叉（SCO）现象是指一些基于d6 - d9过渡金属配合物的材料中由熵驱动的自旋转变。虽然其分子起源已为人熟知，但复杂的SCO行为越来越普遍，其中自旋转变会与例如相变、溶剂吸收/解吸或有序 - 无序过程同时发生。对此类情况的计算建模具有挑战性，因为它需要固态下准确的自旋态能量。密度泛函理论（DFT）是最佳框架，但大多数DFT泛函无法平衡自旋态能量。虽然一些杂化泛函表现更好，但对于中等规模系统的固态极小值搜索来说，它们仍然过于昂贵。最佳替代方案是用哈伯德型校正（DFT + U）来修饰廉价的局域（LDA）或半局域（GGA）DFT泛函。然而，由于缺乏参考值，且从头算参数化方法效果不佳，U的参数化并不直接。此外，SCO配合物在转变时会发生显著的结构变化，因此分子内和分子间相互作用可能在稳定任何一种自旋态中起重要作用。结果，U参数强烈依赖于所使用的色散校正方案。在本文中，我们在使用D3和D3 - BJ色散校正时，对九种已报道的SCO化合物（五种基于FeII，1 - 5，四种基于FeIII，6 - 9）的U进行了参数化。我们分析了色散校正处理对SCO能量学、结构和晶胞尺寸的影响。每种金属离子类型（FeII与FeIII）以及色散校正方案（D3与D3 - BJ）的平均U值都不同，但它们都显示出出色的转移性，平均绝对误差（MAE）低于化学精度（即MAE <4 kJ mol-1）。这使得能够更好地描述含FeII和FeIII离子材料中的SCO过程，更普遍地说，能够更好地描述自旋态能量学。
