Lochan Rohini C, Head-Gordon Martin
Department of Chemistry, University of California, Berkeley, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, California 94720, USA.
J Chem Phys. 2007 Apr 28;126(16):164101. doi: 10.1063/1.2718952.
Coupled-cluster methods based on Brueckner orbitals are well known to resolve the problems of symmetry breaking and spin contamination that are often associated with Hartree-Fock orbitals. However, their computational cost is large enough to prevent application to large molecules. Here the authors present a simple approximation where the orbitals are optimized with the mean-field energy plus a correlation energy taken as the opposite-spin component of the second-order many-body correlation energy, scaled by an empirically chosen parameter (recommended as 1.2 for general applications). This "optimized second-order opposite-spin" (abbreviated as O2) method requires fourth-order computation on each orbital iteration. O2 is shown to yield predictions of structure and frequencies for closed-shell molecules that are very similar to scaled second-order Moller-Plesset methods. However, it yields substantial improvements for open-shell molecules, where problems with spin contamination and symmetry breaking are shown to be greatly reduced.
基于布吕克纳轨道的耦合簇方法，众所周知能够解决常与哈特里 - 福克轨道相关的对称性破缺和自旋污染问题。然而，其计算成本过高，以至于无法应用于大分子。在此，作者提出一种简单的近似方法，其中轨道通过平均场能量加上作为二阶多体关联能量的反自旋分量的关联能量来优化，并按经验选择的参数（一般应用推荐为1.2）进行缩放。这种“优化二阶反自旋”（简称为O2）方法在每次轨道迭代时需要进行四阶计算。结果表明，O2对于闭壳层分子的结构和频率预测与缩放后的二阶莫勒 - 普莱塞特方法非常相似。然而，对于开壳层分子，它有显著改进，其中自旋污染和对称性破缺问题被证明大大减少。
