多参考耦合簇研究 C2B 自由基中的对称性破缺。

Multireference coupled-cluster study of the symmetry breaking in the C2B radical.

机构信息

Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, Canada.

出版信息

J Chem Phys. 2011 Feb 21;134(7):074301. doi: 10.1063/1.3554210.

Abstract

The potential energy surfaces (PESs) for both the ground and the excited electronic states of the C(2)B radical are investigated using various multireference (MR) coupled-cluster (CC) approaches. In the ground state case we employ the reduced MR (RMR) CC approach with singles (S) and doubles (D), the RMR CCSD method, as well as its RMR CCSD(T) version corrected for secondary triples, relying on various model spaces and basis sets. The reliability of this approach is also tested against the benchmark full configuration interaction results obtained for a small Dunning-Hay (DH) basis set. The results imply a clear preference for a cyclic structure which, however, breaks the C(2v) symmetry. This symmetry breaking manifests itself strongly at the level of the independent particle model, as represented by the restricted open-shell Hartree-Fock approximation, but the tendency toward symmetry breaking diminishes with the increasing size of the basis set employed as well as with the enhanced account of the correlation effects. It is likely to disappear in the complete basis set limit. The general model space CCSD method is then used to compute vertical excitation energies for a number of excited states as well as the cuts of the PES as the boron atom moves around the C(2) fragment. These results also explain why no symmetry breaking is found when relying on a spin contaminated unrestricted Hartree-Fock reference, as in the UMP2 method.

摘要

使用各种多参考（MR）耦合簇（CC）方法研究了 C(2)B 自由基的基态和激发电子态的势能面（PES）。在基态情况下，我们采用了简化多参考（RMR）CC 方法，包括单激发（S）和双激发（D），RMR CCSD 方法，以及其针对次级三重态进行修正的 RMR CCSD(T)版本，这些方法都依赖于各种模型空间和基组。该方法的可靠性还通过针对较小的 Dunning-Hay（DH）基组获得的基准全组态相互作用结果进行了测试。结果表明，明显倾向于具有循环结构的体系，但是这种结构打破了 C(2v)对称性。这种对称性破坏在独立粒子模型的水平上表现得非常强烈，这体现在受限开壳 Hartree-Fock 近似中，但是随着基组的增大以及相关性效应的增强，对称性破坏的趋势会减小。这种趋势可能会在完全基组极限中消失。然后，我们使用通用模型空间 CCSD 方法计算了一些激发态的垂直激发能以及硼原子在 C(2)片段周围移动时 PES 的截面。这些结果也解释了为什么在依赖于自旋污染的非限制 Hartree-Fock 参考（如 UMP2 方法）时，不会发现对称性破坏。