Beijing National Laboratory for Molecular Sciences, Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering, and Center for Computational Science and Engineering, Peking University, Beijing 100871, People's Republic of China.
J Chem Phys. 2012 Jan 14;136(2):024106. doi: 10.1063/1.3672085.
Following the theoretical development of a spin-adapted state-specific multi-reference second-order perturbation theory (SA-SSMRPT2) as expounded in the preceding publication, we discuss here its implementation and the results of its applications to potential energy curves (PECs) of various electronic states of small molecules. In particular, we illustrate its efficacy in states of various spin multiplicities and varying multi-reference character. Both Møller-Plesset (MP) and Epstein-Nesbet (EN) type of partitions have been explored. Also, a straightforward Rayleigh-Schrödinger (RS) and Brillouin-Wigner (BW) version of the SA-SSMRPT2 have been studied. Ground state PECs were computed for singlet states of HF, BH, and H(2)O molecules as well as the doublet state of NH(2) and BeH radicals and compared to corresponding full configuration interaction numbers, which serve as benchmark results. As an extensive application on a production level, the ground state PECs of N(2), a classic example of multiple-bond breaking, were calculated using cc-pVXZ (X = 3,4,5) basis and then extrapolated to obtain estimates of the complete basis set limit. Vibrational energy levels were extracted from these N(2) PECs, which compare favorably to the experimental values. In addition, extensive studies were also carried out on PECs of the seven low-lying excited states of the N(2) molecule. Finally, it is shown that the flexibility to relax configuration coefficients in SA-SSMRPT2 helps to provide good descriptions for the avoided crossing between the two lowest (1)Σ states of the LiF molecule. Our results indicate (1) that more studies are needed to draw firm conclusions about the relative efficacies of the MP and EN results and (2) that the RS version works so well as compared to the BW version that the extra computational expenses needed in the later formalism is not warranted.
在前一篇出版物中阐述了自旋自适应态特定多参考二级微扰理论（SA-SSMRPT2）的理论发展之后，我们在这里讨论其实现以及将其应用于各种小分子电子态的势能曲线（PECs）的结果。特别是，我们说明了它在各种自旋多重性和变化的多参考特征的状态中的功效。探索了莫勒-普莱塞特（MP）和爱泼斯坦-内斯贝特（EN）类型的分区。此外，还研究了简单的瑞利-薛定谔（RS）和布里渊-维格纳（BW）版本的 SA-SSMRPT2。计算了 HF、BH 和 H(2)O 分子的单重态的基态 PEC 以及 NH(2)和 BeH 自由基的双重态，并与作为基准结果的相应完全组态相互作用数进行了比较。作为生产水平上的广泛应用，使用 cc-pVXZ（X = 3,4,5）基计算了 N(2)的基态 PEC，然后进行外推以获得完全基集限制的估计值。从这些 N(2)PEC 中提取了振动能级，与实验值相当。此外，还对 N(2)分子的七个低能激发态的 PEC 进行了广泛的研究。最后，表明在 SA-SSMRPT2 中放松构型系数的灵活性有助于提供 LiF 分子的两个最低（1）Σ态之间的避免交叉的良好描述。我们的结果表明：(1) 需要进一步研究，以对 MP 和 EN 结果的相对功效得出明确的结论；(2) RS 版本的效果如此之好，与 BW 版本相比，在后者形式主义中所需的额外计算费用是不值得的。
