Martín Pendás A, Francisco E
Departamento de Química Física y Analítica, Universidad de Oviedo, Oviedo, Spain.
Phys Chem Chem Phys. 2021 Apr 14;23(14):8375-8392. doi: 10.1039/d0cp05946c. Epub 2021 Mar 30.
The theory of open quantum systems (OQSs) is applied to partition the squared spin operator into fragment (local spin) and interfragment (spin-coupling) contributions in a molecular system. An atomic or fragment subsystem is described by a quantum mechanical mixed density operator composed of sectors, characterized by different integer number of electrons that appear with specific probabilities. The OQS fragment spin operators coincide with those defined by Clark and Davidson in their paper on local spins (J. Chem. Phys., 2001, 115, 7382) and are fully consistent with the theory of local operators by Stollhoff and Fulde (J. Chem. Phys., 1980, 73, 4548). OQSs provide a unique way to rationalize the non-zero values of local spins found in closed-shell molecules, a fact that has led to a large number of modified definitions being proposed, which we show suffer from inconsistencies. The OQS viewpoint makes it easy to build models for localized and itinerant spins. These models are used to classify possible local spin arrangements. The role of electron correlation is also studied through the analysis of the Hubbard Hamiltonian in small chains. Local spins result from a game played differently by localized and delocalized electrons. A number of examples exemplifying the ability of the OQS local spin perspective to uncover simple chemical patterns are examined.
开放量子系统（OQSs）理论被应用于将分子系统中的平方自旋算符划分为片段（局域自旋）和片段间（自旋耦合）贡献。一个原子或片段子系统由一个由扇区组成的量子力学混合密度算符描述，这些扇区以不同的整数电子数为特征，并以特定概率出现。OQS片段自旋算符与Clark和Davidson在他们关于局域自旋的论文（《化学物理杂志》，2001年，115卷，7382页）中定义的算符一致，并且与Stollhoff和Fulde的局域算符理论（《化学物理杂志》，1980年，73卷，4548页）完全一致。OQS为合理化闭壳层分子中发现的非零局域自旋值提供了一种独特的方法，这一事实导致大量修改后的定义被提出，但我们表明这些定义存在不一致性。OQS观点使得构建局域和巡游自旋的模型变得容易。这些模型用于对可能的局域自旋排列进行分类。还通过对小链中的哈伯德哈密顿量的分析来研究电子关联的作用。局域自旋源于局域化和离域化电子以不同方式进行的一场“游戏”。研究了一些示例，这些示例体现了OQS局域自旋观点揭示简单化学模式的能力。
