Bao Jie J, Gagliardi Laura, Truhlar Donald G
Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-043, USA.
Phys Chem Chem Phys. 2017 Nov 15;19(44):30089-30096. doi: 10.1039/c7cp05156e.
Multiconfiguration pair-density functional theory (MC-PDFT) is a post multiconfiguration self-consistent field (MCSCF) method with similar performance to complete active space second-order perturbation theory (CASPT2) but with greater computational efficiency. Cyano radical (CN) is a molecule whose spectrum is well established from experiments and whose excitation energies have been used as a testing ground for theoretical methods to treat excited states of open-shell systems, which are harder and much less studied than excitation energies of closed-shell singlets. In the present work, we studied the adiabatic excitation energies of CN with MC-PDFT. Then we compared this multireference (MR) method to some single-reference (SR) methods, including time-dependent density functional theory (TDDFT) and completely renormalized equation-of-motion coupled-cluster theory with singles, doubles and noniterative triples [CR-EOM-CCSD(T)]; we also compared to some other MR methods, including configuration interaction singles and doubles (MR-CISD) and multistate CASPT2 (MS-CASPT2). Through a comparison between SR and MR methods, we achieved a better appreciation of the need to use MR methods to accurately describe higher excited states, and we found that among the MR methods, MC-PDFT stands out for its accuracy for the first four states out of the five doublet states studied this paper; this shows its efficiency for calculating doublet excited states.
多组态对密度泛函理论（MC-PDFT）是一种后多组态自洽场（MCSCF）方法，其性能与完全活性空间二阶微扰理论（CASPT2）相似，但计算效率更高。氰基自由基（CN）是一种分子，其光谱已通过实验得到充分确立，其激发能已被用作处理开壳层体系激发态的理论方法的测试平台，与闭壳层单重态的激发能相比，开壳层体系的激发能更难研究且研究较少。在本工作中，我们用MC-PDFT研究了CN的绝热激发能。然后我们将这种多参考（MR）方法与一些单参考（SR）方法进行了比较，包括含时密度泛函理论（TDDFT）和带单、双激发以及非迭代三激发的完全重整化运动方程耦合簇理论[CR-EOM-CCSD(T)]；我们还与一些其他的MR方法进行了比较，包括组态相互作用单双激发（MR-CISD）和多态CASPT2（MS-CASPT2）。通过SR方法和MR方法之间的比较，我们更好地认识到使用MR方法准确描述高激发态的必要性，并且我们发现，在MR方法中，MC-PDFT在所研究的五个双重态中的前四个态的准确性方面表现突出；这表明了它在计算双重态激发态方面的效率。
