Inclusion of selected higher excitations involving active orbitals in the state-specific multireference coupled-cluster theory.
Inclusion of selected higher excitations involving active orbitals in the state-specific multireference coupled-cluster theory.
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DOI：
10.1063/1.3515478
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发表时间：
2010-12
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影响因子：
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通讯作者：
Sanghamitra Das;Mihály Kállay;D. Mukherjee
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文献类型：
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作者：
Sanghamitra Das;Mihály Kállay;D. Mukherjee
Mukherjee等人[J. Chem. Phys. 110，6171（1999）]提出的母态特异多参考耦合团簇（SS-MRCC）理论，虽然具有严格的尺寸扩展性，并且与定域轨道的尺寸一致，但在最小截断方案中，即在单和双（SD）水平（SS-MRCCSD）上，存在一些缺陷。SS-MRCCSD不涉及所有模型函数与属于非收缩多组态CISD空间的给定虚函数的直接耦合。它也不涉及，即使在一个集群操作T（μ）的线性功率，直接耦合的虚拟功能χ（1（μ）），这是高达双激发相对于一个模型函数φ（μ）的MRCISD空间的其他虚拟功能，可以产生的三重和四重激发φ（μ）。我们认为，在每个φ（μ）中加入最多包含两个惰性轨道激发的三重和四重激发，可以改善三重和四重激发虚函数与单重和双重激发虚函数耦合不完全的缺点.我们的SS-MRCC理论的这种扩展的分析（我们称之为SS-MRCCSDtq）仍然会遗漏模型函数{φ（λ），λ <$μ}的流形与单激发和双激发虚函数的直接耦合。然而，这种影响预计将不太重要，比缺乏更完整的虚拟空间耦合，这些功能是许多，建议新的方法显着改进的计划。在涉及单键、双键和三键解离的小分子势能面上的优异结果充分证实了我们的期望。
The parent state-specific multireference coupled-cluster (SS-MRCC) theory proposed by Mukherjee et al. [J. Chem. Phys. 110, 6171 (1999)], though rigorously size-extensive and also size-consistent with localized orbitals, has some deficiencies in the minimal truncation scheme, viz. at the singles and doubles (SD) level (SS-MRCCSD). SS-MRCCSD does not involve the direct coupling of all the model functions with a given virtual function belonging to the uncontracted multiconfiguration CISD space. It also does not involve, even in the linear power of a cluster operator T(μ), the direct coupling of the virtual functions χ(l(μ)), which are up to doubly excited with respect to a model function φ(μ) to the other virtual functions of the MRCISD space which can be generated by triple and quadruple excitations from φ(μ). We argue that inclusion of a selection of triples and quadruples involving at most two inactive orbital excitations from every φ(μ) would ameliorate the shortcoming of the incomplete coupling of the triply and quadruply excited virtual functions which can couple with the singly and doubly excited ones. This extended ansatz for our SS-MRCC theory, to be called SS-MRCCSDtq by us, would still miss the direct coupling of the manifold of the model functions {φ(λ),λ ≠ μ} to singly and doubly excited virtual functions. However, this effect is expected to be less significant than the lack of the more complete virtual space couplings, these functions being many more numerous, suggesting the new methods to be significantly improved schemes. Excellent results on the potential energy surfaces of small molecules involving single, double, and triple bond dissociation bear out our expectations fully.