Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
Laboratory of Structural and Computational Physical-Chemistry for Nanosciences and QSAR, Biology-Chemistry Department, Faculty of Chemistry, Biology, Geography, West University of Timisoara, Str. Pestalozzi No. 16, 300115 Timisoara, Romania.
Int J Mol Sci. 2021 Aug 19;22(16):8953. doi: 10.3390/ijms22168953.
In this paper, we present a formulation of highly correlated Fock-space multi-reference coupled-cluster (FSMRCC) methods, including approximate triples on top of the FSMRCC with singles and doubles, which correct the electron affinities by at least at third and up to the fourth order in perturbation. We discuss various partial fourth-order schemes, which are reliable and yet computationally more efficient than the full fourth-order triples scheme. The third-order scheme is called MRCCSD+T(3). We present two approximate fourth-order schemes, MRCCSD+T-a(4) and MRCCSD+T(4). The results that are presented allow one to choose an appropriate fourth-order scheme, which is less expensive and right for the problem. All these schemes are based on the effective Hamiltonian scheme, and provide a direct calculation of the vertical electron affinities. We apply these schemes to a prototype Li molecule, using four different basis sets, as well as BeO and CH. We have calculated the vertical electron affinities of Li at the geometry of the neutral Li molecule. We also present the vertical ionization potentials of the Li anion at the geometry of the anion ground state. We have also shown how to calculate adiabatic electron affinity, though in that case we lose the advantages of direct calculation. BeO has been examined in two basis sets. For CH, four different basis sets have been used. We have presented the partial fourth-order schemes to the EA in all the basis sets. The results are analyzed to illustrate the importance of triples, as well as highlight computationally efficient partial fourth-order schemes. The choice of the basis set on the electron affinity calculation is also emphasized. Comparisons with available experimental and theoretical results are presented. The general fourth-order schemes, which are conceptually equivalent with the Fock-space multi-reference coupled-cluster singles, doubles, and triplets (MRCCSD+T) methods, based on bondonic formalism, are also presented here in a composed way, for quantum electronic affinity.
在本文中，我们提出了一种高度相关的 Fock 空间多参考耦合簇（FSMRCC）方法的表述，包括在 FSMRCC 之上的近似三重态和单重态和双重态，该方法通过至少三阶和至多四阶微扰来校正电子亲和能。我们讨论了各种部分四阶方案，这些方案既可靠又比全四阶三重态方案更具计算效率。三阶方案称为 MRCCSD+T(3)。我们提出了两种近似四阶方案，MRCCSD+T-a(4)和 MRCCSD+T(4)。所呈现的结果允许选择一个适当的四阶方案，该方案的成本更低，适用于该问题。所有这些方案都是基于有效哈密顿量方案，并且提供了垂直电子亲和能的直接计算。我们将这些方案应用于 Li 分子的原型，使用了四种不同的基组，以及 BeO 和 CH。我们已经计算了中性 Li 分子几何形状下 Li 的垂直电子亲和能。我们还给出了 Li 阴离子在阴离子基态几何形状下的垂直电离势。我们还展示了如何计算绝热电子亲和能，尽管在这种情况下，我们会失去直接计算的优势。BeO 在两个基组中进行了检查。对于 CH，使用了四个不同的基组。我们在所有基组中都给出了 EA 的部分四阶方案。结果进行了分析，以说明三重态的重要性，并突出计算效率高的部分四阶方案。还强调了电子亲和能计算中基组的选择。还给出了与现有实验和理论结果的比较。基于邦顿形式主义的、概念上等效于 Fock 空间多参考耦合簇单重态、双重态和三重态（MRCCSD+T）方法的一般四阶方案，也以组合的方式呈现出来，用于量子电子亲和能。
