对称性破缺方法在交换相互作用中的失效
Breakdown of broken-symmetry approach to exchange interaction
AI总结：
本文证明对称性破缺方法在评估Heisenberg交换参数时存在内在误差，该误差与轨道共价性相关，并提出多组态扩展作为解决方案。
摘要：
对称性破缺（BS）方法被广泛用于评估Heisenberg交换参数，主要与DFT计算结合使用。对于许多磁性材料，BS-DFT计算能给出合理的交换参数估计，但也存在系统性失效的报道。尽管后者被归因于近似交换关联泛函的缺陷，我们在此通过处理一个简单模型系统证明，对称性破缺方法本身存在严重问题。详细分析阐明了对称性破缺处理低自旋态的内在问题。特别地，BS计算中交换参数的误差与磁性轨道和桥接轨道之间的共价性程度成正比。这是由于BS行列式对多组态形式施加的约束，这一特征也存在于其他单参考方法中。为克服单行列式BS方法的这一内在缺陷，我们提出将其扩展为最小多组态版本。
英文摘要：
Broken-symmetry (BS) approaches are widely employed to evaluate Heisenberg exchange parameters, primarily in combination with DFT calculations. For many magnetic materials, BS-DFT calculations give reasonable estimations of exchange parameters, although systematic failures have also been reported. While the latter were attributed to deficiencies of approximate exchange-correlation functional, we prove here by treating a simple model system that the broken-symmetry methodology has serious problems. Detailed analysis clarifies the intrinsic issue with the broken-symmetry treatment of low-spin states. It shows, in particular, that the error in the BS calculation of exchange parameter scales with the degree of covalency between the magnetic and the bridging orbitals. This is due to the constraint on the form of multiconfigurational state imposed by the BS determinant, a feature common to other single-reference methods too. As a possible tool to overcome this intrinsic drawback of single-determinant BS approaches, we propose their extension to a minimal multiconfigurational version.
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