具有不同参考函数的不同运动方程耦合簇方法：甲酰基自由基。

Different equation-of-motion coupled cluster methods with different reference functions: the formyl radical.

作者信息

Kuś Tomasz, Bartlett Rodney J

机构信息

Quantum Theory Project, Department of Chemistry, University of Florida Gainesville, Florida 32611, USA.

出版信息

J Chem Phys. 2008 Sep 14;129(10):104301. doi: 10.1063/1.2975205.

DOI:10.1063/1.2975205

PMID:19044907

Abstract

The doublet and quartet excited states of the formyl radical have been studied by the equation-of-motion (EOM) coupled cluster (CC) method. The S(z) spin-conserving singles and doubles (EOM-EE-CCSD) and singles, doubles, and triples (EOM-EE-CCSDT) approaches, as well as the spin-flipped singles and doubles (EOM-SF-CCSD) method have been applied, subject to unrestricted Hartree-Fock (HF), restricted open-shell HF, and quasirestricted HF references. The structural parameters, vertical and adiabatic excitation energies, and harmonic vibrational frequencies have been calculated. The issue of the reference function choice for the spin-flipped (SF) method and its impact on the results has been discussed using the experimental data and theoretical results available. The results show that if the appropriate reference function is chosen so that target states differ from the reference by only single excitations, then EOM-EE-CCSD and EOM-SF-CCSD methods give a very good description of the excited states. For the states that have a non-negligible contribution of the doubly excited configurations one is able to use the SF method with such a reference function, that in most cases the performance of the EOM-SF-CCSD method is better than that of the EOM-EE-CCSD approach.

摘要

通过运动方程（EOM）耦合簇（CC）方法研究了甲酰基自由基的双重态和四重态激发态。应用了S(z)自旋守恒单双激发（EOM-EE-CCSD）和单双三激发（EOM-EE-CCSDT）方法，以及自旋翻转单双激发（EOM-SF-CCSD）方法，计算时采用了无限制Hartree-Fock（HF）、限制开壳层HF和准限制HF参考态。计算了结构参数、垂直和绝热激发能以及谐振动频率。利用现有实验数据和理论结果，讨论了自旋翻转（SF）方法的参考函数选择问题及其对结果的影响。结果表明，如果选择合适的参考函数，使目标态与参考态仅通过单激发不同，那么EOM-EE-CCSD和EOM-SF-CCSD方法能很好地描述激发态。对于双激发组态贡献不可忽略的态，能够使用具有这种参考函数的SF方法，在大多数情况下，EOM-SF-CCSD方法的性能优于EOM-EE-CCSD方法。