Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany.
Interdisciplinary Center for Scientific Computing, Ruprecht-Karls University, Heidelberg, Germany.
J Comput Chem. 2021 Jul 30;42(20):1402-1418. doi: 10.1002/jcc.26552. Epub 2021 May 16.
Excitonic coupling plays a key role for the understanding of excitonic energy transport (EET) in, for example, organic photovoltaics. However, the calculation of realistic systems is often beyond the applicability range of accurate wavefunction methods so that lower-scaling semi-empirical methods are used to model EET events. In the present work, the distance and angle dependence of excitonic couplings of dimers of selected organic molecules are evaluated for the semi-empirical long-range corrected density functional based tight binding (LC-DFTB) method and spin opposite scaled second order approximate coupled cluster singles and doubles (SOS-CC2). While semi-empirically scaled methods can lead to slightly increased deviations for excitation energies, the excitonic couplings and their dependence on the dimer geometry are reproduced. LC-DFTB yields a similar accuracy range as density-functional theory (DFT) employing the ωB97X functional while the computation time is reduced by several orders of magnitude. The dependence of the exchange contributions to the excitonic couplings on the dimer geometry is analyzed assessing the calculation of Coulombic excitonic couplings from monomer local excited states only, which reduces the computational effort significantly. The present work is a necessary first step toward the simulation of excitonic energy transport using semi-empirical methods.
激子耦合对于理解激子能量转移（EET）起着关键作用，例如在有机光伏中。然而，对于实际系统的计算通常超出了精确波函数方法的适用性范围，因此需要使用低标度的半经验方法来模拟 EET 事件。在本工作中，使用半经验的长程修正密度泛函基紧束缚（LC-DFTB）方法和自旋相反标度二阶近似耦合簇单双激发（SOS-CC2）方法评估了选定有机分子二聚体的激子耦合的距离和角度依赖性。虽然半经验标度方法可能会导致激发能的略微增加偏差，但激子耦合及其对二聚体几何形状的依赖性得到了重现。LC-DFTB 的精度范围与采用 ωB97X 泛函的密度泛函理论（DFT）相当，而计算时间则减少了几个数量级。分析了激子耦合中交换贡献对二聚体几何形状的依赖性，评估了仅从单体局域激发态计算库仑激子耦合的计算，这大大减少了计算工作量。本工作是使用半经验方法模拟激子能量转移的必要的第一步。
