Predicting exchange coupling constants in frustrated molecular magnets using density functional theory.
Predicting exchange coupling constants in frustrated molecular magnets using density functional theory.
复制标题
使用密度泛函理论预测受挫分子磁体中的交换耦合常数。
作者：
I. Rudra;Qin Wu;T. Van Voorhis
我们使用各种理论技术研究了具有强自旋挫败的多核过渡金属簇中的海森堡交换耦合。我们展示了三核 Cr(III) 分子、四核 Fe(III) 配合物和八核 Fe(III) 分子磁体的结果。我们使用标准破缺对称（BS）技术和最近开发的约束密度泛函理论（C-DFT）方法探索这些系统中交换耦合的物理原理。计算表明，金属中心上局部自旋矩的预期图像是合适的，并且在每种情况下，C-DFT 预测的耦合常数值与实验非常吻合。此外，我们证明给定簇的所有 C-DFT 自旋状态都可以由单个海森堡哈密顿量合理地描述。这些发现很重要，部分原因是标准 BS 计算与许多关键点的实验相冲突。例如，BS-DFT 预测 Cr(III) 簇的双重态（而不是四重态）基态，而对于 Fe(III) 配合物，BS-DFT 预测一些交换耦合是铁磁性的，而实验得出的耦合都是反铁磁性的。此外，对于 BS-DFT，最佳拟合交换参数可能很大程度上取决于所选的自旋配置集。例如，通过选择 Ms 更接近 Ms(max) 的配置，BS-DFT 耦合通常可以在某种程度上更接近 C-DFT 和实验结果。因此，在这些情况下，我们的结果始终支持实验结果。
We study the Heisenberg exchange couplings in polynuclear transition-metal clusters with strong spin frustration using a variety of theoretical techniques. We present results for a trinuclear Cr(III) molecule, a tetranuclear Fe(III) complex, and an octanuclear Fe(III) molecular magnet. We explore the physics of the exchange couplings in these systems using standard broken-symmetry (BS) techniques and a more recently developed constrained density functional theory (C-DFT) approach. The calculations show that the expected picture of localized spin moments on the metal centers is appropriate, and in each case C-DFT predicts coupling constant values in good agreement with experiment. Furthermore, we demonstrate that all of the C-DFT spin states for a given cluster can be reasonably described by a single Heisenberg Hamiltonian. These findings are significant in part because standard BS calculations are in conflict with the experiments on a number of key points. For example, BS-DFT predicts a doublet (rather than quartet) ground state for the Cr(III) cluster while for the Fe(III) complexes BS-DFT predicts some of the exchange couplings to be ferromagnetic whereas the experimentally derived couplings are all antiferromagnetic. Furthermore, for BS-DFT the best-fit exchange parameters can depend significantly on the set of spin configurations chosen. For example, by choosing configurations with Ms closer to Ms(max) the BS-DFT couplings can typically be made somewhat closer to the C-DFT and experimental results. Thus, in these cases, our results consistently support the experimental findings.