Theoretical study of the valence pi->pi* excited states of polyacenes: Benzene and naphthalene
Theoretical study of the valence pi->pi* excited states of polyacenes: Benzene and naphthalene
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DOI：
10.1063/1.471286
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发表时间：
1996-04-22
影响因子：
4.4
通讯作者：
Hirao, K
中科院分区：
文献类型：
--
作者：
Hashimoto, T;Nakano, H;Hirao, K
采用具有完全活性空间自洽场（CASSCF）参考函数的多参考微扰理论，研究了苯和萘的价π--> π * 态.具有π价活性轨道的CASSCF的本征矢和本征值在很好的近似下满足交替烃的配对性质。利用交替对称性将多并苯的激发态分为共价负态和离子正态。本理论满意地描述了低价π-> π ~* 激发态的有序性。本方法的总体准确性令人惊讶地高。我们能够预测的价态激发能的精度为0.27 eV的单重态u和0.52 eV或更好的单重态g状态的萘。我们预测的三重态光谱提供了一个一致的归属萘的三重态-三重态吸收光谱。对于苯，我们能够预测的价态激发能与约0.29 eV的精度。就电子关联而言，共价负态和离子正态表现出不同的行为。离子正态主要是由单激发，但共价负态包括大部分的双激发配置。共价负态总是比相应的离子正态能量低。这对于三重态是正确的。微扰理论引入的动态σ-π极化效应对于离子正态是显著的，而对于共价激发态的σ-π极化效应通常与共价基态的σ-π极化效应具有相同的量级。参考函数有效空间的扩大代表了离子态描述的巨大改进。本文的方法对理解和预测交替烃的实验数据具有重要价值。(C)1996年美国物理学会。
Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions was applied to the study of the valence pi-->pi* states of benzene and naphthalene. The eigenvectors and eigenvalues of CASSCF with valence pi active orbitals satisfy pairing properties for the alternant hydrocarbons to a good approximation. The excited states of polyacenes are classified into the covalent minus states and ionic plus states with the use of the alternancy symmetry. The present theory satisfactorily describes the ordering of low-lying valence pi-->pi* excited states. The overall accuracy of the present approach is surprisingly high. We were able to predict the valence excitation energies with an accuracy of 0.27 eV for singlet u states and of 0.52 eV or better for singlet g states of naphthalene. Our predicted triplet states spectrum provides a consistent assignment of the triplet-triplet absorption spectrum of naphthalene. For benzene we were able to predict the valence excitation energy with an accuracy of about 0.29 eV. The covalent minus states and ionic plus states exhibit different behavior as far as the electron correlation is concerned. The ionic plus states are dominated by the single excitations but covalent minus states include a large fraction of doubly excited configurations. The covalent minus states always give lower energy than the corresponding ionic plus states. This is true for triplet states. The dynamic sigma-pi polarization effects introduced by perturbation theory are significant for the ionic plus states while those on covalent excited states are usually of the same order as in the covalent ground state. The enlargement of the active space of the reference functions represents a great improvement of the description of the ionic states. The present approach with the pairing properties has proved to be of great value in understanding and predicting the experimental data of the alternant hydrocarbons. (C) 1996 American Institute of Physics.