Chattopadhyay Sudip, Chaudhuri Rajat K, Mahapatra Uttam Sinha
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, 711103, India.
J Comput Chem. 2015 May 5;36(12):907-25. doi: 10.1002/jcc.23873. Epub 2015 Mar 5.
Adaptation of improved virtual orbitals (IVOs) in state-specific multireference perturbation theory using Møller-Plesset multipartitioning of the Hamiltonian (IVO-SSMRPT) is examined in which the IVO-complete active space configuration interaction (CASCI) is used as an inexpensive alternative to the more involved CAS-self-consistent field (CASSCF) orbitals. Unlike the CASSCF approach, IVO-CASCI does not bear tedious and costly iterations beyond those in the initial SCF calculation. The IVO-SSMRPT is intruder-free, and explicitly size-extensive. In the present preliminary study, the IVO-SSMRPT method which relies on a small reference space is applied to study potential energy surfaces (PES) of the ground state of challenging, multiconfigurational F2 , Be2 , and N2 molecules. These systems provide a serious challenge to any ab initio methodology due to the presence of an intricate interplay of nondynamical and dynamical correlations to the entire PES. The quality of the computed PES has been judged by extracting spectroscopic parameters and vibrational levels. The reported results illustrate that the IVO-SSMRPT method has a potential to yield accuracies as good as the CASSCF-SSMRPT one with reduced computational labor. Even with small reference spaces, our estimates demonstrate a good agreement with the available experimental values, and some benchmark computations. The blend of accuracy and low computational cost of IVO-SSMRPT should deserve future attention for the accurate treatment of electronic states of small to large molecular systems for which the wavefunction is characterized by various configurations.
研究了在使用哈密顿量的莫勒-普莱塞特多划分的态特定多参考微扰理论（IVO-SSMRPT）中改进虚拟轨道（IVO）的适配情况，其中IVO-完全活性空间组态相互作用（CASCI）被用作比更复杂的CAS自洽场（CASSCF）轨道更廉价的替代方法。与CASSCF方法不同，IVO-CASCI除了初始SCF计算中的迭代之外，没有冗长且昂贵的迭代。IVO-SSMRPT没有侵入态，并且显式具有规模扩展性。在当前的初步研究中，依赖于小参考空间的IVO-SSMRPT方法被应用于研究具有挑战性的多组态F2、Be2和N2分子基态的势能面（PES）。由于在整个PES中存在非动态和动态相关性的复杂相互作用，这些系统对任何从头算方法都构成了严峻挑战。通过提取光谱参数和振动能级来判断计算得到的PES的质量。报告的结果表明，IVO-SSMRPT方法有可能在减少计算量的情况下产生与CASSCF-SSMRPT方法相当的精度。即使参考空间很小，我们的估计也与可用的实验值以及一些基准计算结果吻合良好。IVO-SSMRPT的精度和低计算成本的结合，对于准确处理波函数具有各种组态特征的从小分子到大分子系统的电子态，应该值得未来关注。
