Coupled cluster valence bond (CCVB) is a simple electronic structure method based on a perfect pairing (PP) reference with 2-pair recouplings for strong electron correlation problems. CCVB is spin-pure, size-consistent, and can exactly (in its active space) separate any molecule into atoms for which unrestricted Hartree-Fock (UHF) at dissociation is the sum of the ground state UHF energies of the atoms. However CCVB is far from a complete description of strong correlations. Its first failure to exactly describe spin-recouplings arises at the level of 3 electron pairs, such as the recoupling of 3 triplet oxygen atoms in the dissociation of singlet ozone. Such situations are often associated with spin frustration. To address this limitation, an extension of CCVB, termed CCVB+i3, is reported here that includes an independent (i) amplitude approximation to the 3-pair recouplings. CCVB+i3 thereby has the same basic computational requirements as those of CCVB, which has previously been shown to be an efficient method. CCVB+i3 correctly separates molecules that CCVB cannot. As a by-product, an independent 2-pair amplitude approximation to CCVB, called PP+i2, is also defined. Remarkably, PP+i2 can also correctly separate systems that CCVB cannot. CCVB+i3 is validated on the symmetric dissociation of D_3h ozone. CCVB+i3 is then used to explore the role of 3-pair recouplings in an [Fe₄S₄(SCH₃)₄]²⁻ cluster that has been used to model the iron-sulfur core of [Fe₄S₄] ferredoxins. Using localized PP orbitals, such recouplings are demonstrated to be large in some low-lying singlet excited states of the cluster. Significant 3 pair recoupling amplitudes include the usual triangular motif associated with spin frustration and other geometric arrangements of the 3 entangled pairs across the 4 iron centers.

中文翻译：

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耦合簇价键理论中的独立振幅近似：结合3-电子对相关性并将其应用于铁氧还蛋白型四金属铁硫簇的低激发态的自旋失意

耦合簇价键（CCVB）是一种基于电子对问题的基于2对耦合的完全配对（PP）参考的简单电子结构方法。CCVB是自旋纯净的，尺寸一致的，并且可以精确地（在其活动空间中）将任何分子分离为原子，对于这些原子，解离时的无限制Hartree-Fock（UHF）是原子的基态UHF能量之和。但是，CCVB远不是完整的强相关性描述。它的第一个未能准确描述自旋耦合的失败源于3个电子对的水平，例如单线态臭氧解离中3个三重态氧原子的重新耦合。这种情况通常与自旋沮丧有关。为了解决此限制，我们对CCVB进行了扩展，称为CCVB + i3，此处报道的信号包括一个独立的（i）3对耦合的幅度近似值。CCVB + i3因此具有与CCVB相同的基本计算要求，而CCVB + i3先前已被证明是一种有效的方法。CCVB + i3可以正确分离CCVB无法分离的分子。作为副产品，还定义了CCVB的独立2对幅度近似值，称为PP + i2。值得注意的是，PP + i2还可以正确分离CCVB无法分离的系统。CCVB + i3在D的对称解离上得到验证 值得注意的是，PP + i2还可以正确分离CCVB无法分离的系统。CCVB + i3在D的对称解离上得到验证 值得注意的是，PP + i2还可以正确分离CCVB无法分离的系统。CCVB + i3在D的对称解离中得到验证臭氧_3小时。然后使用CCVB + i3探索3对偶联在[Fe ₄ S ₄（SCH ₃）₄ ] ²⁻簇中的作用，该簇已用于模拟[Fe ₄ S ₄ ]铁氧还蛋白的铁硫核。使用局部PP轨道，这种耦合在簇的某些低洼单重态激发态中被证明是很大的。3对重要的耦合幅度包括与旋转挫折感相关的常见三角形图案，以及3个纠缠对在4个铁心上的其他几何排列。