Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of the Ministry of Education (MOE), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China.
J Chem Phys. 2011 Oct 7;135(13):134117. doi: 10.1063/1.3643838.
Two lowest-lying excited singlets with B(u) symmetry of all-trans-oligoenes, the well-known ionic 1(1)B(u)(+) state as well as the "hidden" ionic-covalent-mixed 1(1)B(u)(-) state, are calculated within both the Pariser-Parr-Pople (PPP) model at full configuration interaction (FCI) level and ab initio methods. The vertical excitation energies as well as wavefunctions from PPP-FCI calculations are found to be in good agreement with those from high-level multi-reference methods, such as multi-reference complete active space self-consistent field (CASSCF) with second order perturbative corrections (CASPT2), multi-reference Møller-Plesset perturbation theory (MRMP), and complete active space valence bond theory (CASVB). The oscillator strengths from PPP calculation are in good agreement with spectroscopy experiments. The relatively small oscillator strength of 1(1)B(u)(-) is due to the approximate electron-hole symmetry of this state. In addition, the bond lengths in both states are found to show remarkable relativity with the bond orders calculated with ground state geometries, which suggests a possible strategy for initial guess in geometry optimization of excited states.
全反式聚烯的 B(u) 对称的两个最低激发单线态，即众所周知的离子 1(1)B(u)(+)态以及“隐藏”的离子-共价混合 1(1)B(u)(-)态，在 Pariser-Parr-Pople (PPP) 模型和从头算方法的全组态相互作用 (FCI) 水平上进行了计算。PPP-FCI 计算的垂直激发能和波函数与高水准多参考方法（如多参考完全活性空间自洽场 (CASSCF) 加二阶微扰修正 (CASPT2)、多参考 Møller-Plesset 微扰理论 (MRMP) 和完全活性空间价键理论 (CASVB)）非常吻合。PPP 计算的振子强度与光谱实验吻合良好。1(1)B(u)(-) 态的振子强度较小，这是由于该态的电子-空穴近似对称性所致。此外，两个态中的键长与基态几何结构计算的键序之间存在显著的相关性，这为激发态几何优化的初始猜测提供了一种可能的策略。
