Department of Chemistry, University of Southern California, Los Angeles, California.
J Comput Chem. 2023 Jan 30;44(3):367-380. doi: 10.1002/jcc.26941. Epub 2022 Jun 14.
Low-energy spectra of single-molecule magnets (SMMs) are often described by Heisenberg Hamiltonians. Within this formalism, exchange interactions between magnetic centers determine the ground-state multiplicity and energy separation between the ground and excited states. In this contribution, we extract exchange coupling constants (J) for a set of iron (III) binuclear and tetranuclear complexes from all-electron calculations using non-collinear spin-flip time-dependent density functional theory (NC-SF-TDDFT). For 12 binuclear complexes with J-values ranging from -6 to -132 cm , our benchmark calculations using the short-range hybrid ωPBEh functional and 6-31G(d,p) basis set agree well with the experimentally derived values (mean absolute error of 4.7 cm ). For the tetranuclear SMMs, the computed J constants are within 6 cm from the experimentally derived values. We explore the range of applicability of the Heisenberg model by analyzing bonding patterns in these Fe(III) complexes using natural orbitals (NO), their occupations, and the number of effectively unpaired electrons. The results illustrate the efficiency of the spin-flip protocol for computing the exchange couplings and the utility of the NO analysis in assessing the validity of effective spin Hamiltonians.
单分子磁体（SMMs）的低能谱通常用海森堡哈密顿量来描述。在这个形式体系中，磁中心之间的交换相互作用决定了基态多重性和基态与激发态之间的能量分离。在本研究中，我们从全电子计算中提取了一组铁（III）双核和四核配合物的交换耦合常数（J），使用非共线自旋翻转含时密度泛函理论（NC-SF-TDDFT）。对于 12 个 J 值范围在-6 到-132 cm 的双核配合物，我们使用短程混合 ωPBEh 泛函和 6-31G(d,p)基组的基准计算与实验得出的值吻合良好（平均绝对误差为 4.7 cm ）。对于四核 SMM，计算得到的 J 常数与实验得出的值相差 6 cm 以内。我们通过分析这些 Fe(III)配合物的成键模式，使用自然轨道（NO）、它们的占据数和有效不成对电子数，来探究海森堡模型的适用范围。结果表明，自旋翻转协议在计算交换耦合常数方面非常有效，而 NO 分析在评估有效自旋哈密顿量的有效性方面也非常有用。
