变关联区下的裁剪耦合团簇理论
Tailored Coupled Cluster Theory in Varying Correlation Regimes
摘要：
定制耦合团簇（TCC）方法是一种很有前途的分析方法，它保留了单参考耦合团簇理论的简单性，同时通过从前面的多组态计算获得的振幅引入了多参考波函数。在这里，我们提出了一个详细的分析TCC波函数的基础上模型系统，这需要一个准确的描述静态和动态的相关性。我们调查的可靠性TCC方法相对于精确的波函数。除了电子能量和标准耦合簇诊断中的误差之外，我们利用TCC和全组态相互作用波函数的重叠作为质量度量。我们严格审查的问题，如所需的大小的活动空间，大小一致性，对称性破缺的波函数，和参考波函数上的TCC的依赖。我们观察到，对称性破缺可能引起的误差可以通过采用在有效空间中具有最大权重的行列式作为TCC计算的参考来减轻。我们发现TCC模型是有前途的计算与活跃的轨道空间，其中包括所有的轨道与一个大的单轨道熵，即使活跃的空间变得非常大，然后可能需要现代的活跃空间的方法，不限于相对较少的轨道数。此外，利用大的活性空间可以改善TCC波函数近似并减少尺寸一致性误差，因为高激发决定簇的存在影响活性空间中低激发决定簇的系数的准确性。
英文摘要：
The tailored coupled cluster (TCC) approach is a promising ansatz that preserves the simplicity of single-reference coupled cluster theory, while incorporating a multi-reference wave function through amplitudes obtained from a preceding multi-configurational calculation. Here, we present a detailed analysis of the TCC wave function based on model systems, which require an accurate description of both static and dynamic correlation. We investigate the reliability of the TCC approach with respect to the exact wave function. In addition to the error in the electronic energy and standard coupled cluster diagnostics, we exploit the overlap of TCC and full configuration interaction wave functions as a quality measure. We critically review issues such as the required size of the active space, size-consistency, symmetry breaking in the wave function, and the dependence of TCC on the reference wave function. We observe that possible errors caused by symmetry breaking can be mitigated by employing the determinant with the largest weight in the active space as reference for the TCC calculation. We find the TCC model to be promising in calculations with active orbital spaces which include all orbitals with a large single-orbital entropy, even if the active spaces become very large and then may require modern active-space approaches that are not restricted to comparatively small numbers of orbitals. Furthermore, utilizing large active spaces can improve on the TCC wave-function approximation and reduce the size-consistency error, because the presence of highly excited determinants affects the accuracy of the coefficients of low-excited determinants in the active space.
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