Electronic Structures of Anti-Ferromagnetic Tetraradicals: Ab Initio and Semi -Empirical Studies
Electronic Structures of Anti-Ferromagnetic Tetraradicals: Ab Initio and Semi -Empirical Studies
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反铁磁四自由基的电子结构：从头算和半经验研究
DOI：
10.1021/acs.jctc.6b00103
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发表时间：
2016
影响因子：
5.5
通讯作者：
Liu Chungen
中科院分区：
文献类型：
--
作者：
Zhang Dawei;Liu Chungen
本文用高水平的从头算和半经验方法研究了几种反铁磁四自由基模型系统中最低自旋态的能量关系和电子结构。采用全CI方法（FCI）、完全活性空间二级微扰理论（CASPT 2）和电子价态微扰理论（NEVPT 2）得到了参考结果。通过比较Heisenberg模型和Hubbard模型预测的能量关系与ab initiobenchmarks，对广泛使用的低自旋多自由基反铁磁自旋耦合的Heisenberg模型的准确性进行了仔细的检验.发现电子关联的强度（|u/t|关于反铁磁耦合的自由基中心，可以从强关联区到中等关联区变化，并且可以成为除了自旋多重性之外的另一个自由度。因此，海森伯型模型在强关联区工作得很好，它很好地再现了所有自旋态的能量关系沿着波函数。在适度自旋关联的四体中，原型海森堡模型的结果与多参考电子关联从头算方法的结果严重偏离，而扩展的海森堡模型，包含四体项，可以引入合理的修正，并在此条件下保持其准确性.在弱关联区，海森堡原型模型及其包含高阶修正项的扩展形式都将遇到困难。同时，Hubbard模型从强关联到弱关联都表现出平衡的精度，并且能够定性地再现正确的电子结构，这使得它更适合于多自由基体系中反铁磁耦合的研究.
The energy relationships and electronic structures of the lowest-lying spin states in several anti-ferromagnetic tetraradical model systems are studied with high-levelab initioand semi-empirical methods. The Full-CI method (FCI), the complete active space second-order perturbation theory (CASPT2), and then-electron valence state perturbation theory (NEVPT2) are employed to obtain reference results. By comparing the energy relationships predicted from the Heisenberg and Hubbard models withab initiobenchmarks, the accuracy of the widely used Heisenberg model for anti-ferromagnetic spin-coupling in low-spin polyradicals is cautiously tested in this work. It is found that the strength of electron correlation (|U/t|) concerning anti-ferromagnetically coupled radical centers could range widely from strong to moderate correlation regimes and could become another degree of freedom besides the spin multiplicity. Accordingly, the Heisenberg-type model works well in the regime of strong correlation, which reproduces well the energy relationships along with the wave functions of all the spin states. In moderately spin-correlated tetraradicals, the results of the prototype Heisenberg model deviate severely from those of multi-reference electron correlationab initiomethods, while the extended Heisenberg model, containing four-body terms, can introduce reasonable corrections and maintains its accuracy in this condition. In the weak correlation regime, both the prototype Heisenberg model and its extended forms containing higher-order correction terms will encounter difficulties. Meanwhile, the Hubbard model shows balanced accuracy from strong to weak correlation cases and can reproduce qualitatively correct electronic structures, which makes it more suitable for the study of anti-ferromagnetic coupling in polyradical systems.