The present work investigates the performance of exchange–correlation functionals in the prediction of two-photon absorption (2PA) strengths. For this purpose, we considered six common functionals used for studying 2PA processes and tested these on six organoboron chelates. The set consisted of two semilocal (PBE and BLYP), two hybrid (B3LYP and PBE0), and two range-separated (LC-BLYP and CAM-B3LYP) functionals. The RI-CC2 method was chosen as a reference level and was found to give results consistent with the experimental data that are available for three of the molecules considered. Of the six exchange–correlation functionals studied, only the range-separated functionals predict an ordering of the 2PA strengths that is consistent with experimental and RI-CC2 results. Even though the range-separated functionals predict correct relative trends, the absolute values for the 2PA strengths are underestimated by a factor of 2–6 for the molecules considered. An in-depth analysis, on the basis of the derived generalized few-state model expression for the 2PA strength for a coupled-cluster wave function, reveals that the problem with these functionals can be linked to underestimated excited-state dipole moments and, to a lesser extent, overestimated excitation energies. The semilocal and hybrid functionals exhibit less predictable errors and a variation in the 2PA strengths in disagreement with the reference results. The semilocal and hybrid functionals show smaller average errors than the range-separated functionals, but our analysis reveals that this is due to fortuitous error cancellation between excitation energies and the transition dipole moments. Our results constitute a warning against using currently available exchange–correlation functionals in the prediction of 2PA strengths and highlight the need for functionals that correctly describe the electron density of excited electronic states.

中文翻译：

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基准交换相关函数的性能以预测双光子吸收强度

本工作研究了交换相关函数在预测双光子吸收（2PA）强度方面的性能。为此，我们考虑了用于研究2PA工艺的六个常用功能，并在六个有机硼螯合物上对其进行了测试。该集合由两个半局部（PBE和BLYP），两个混合（B3LYP和PBE0）和两个范围分隔（LC-BLYP和CAM-B3LYP）功能组成。选择RI-CC2方法作为参考水平，发现该方法的结果与所考虑的三种分子可获得的实验数据一致。在研究的六个交换相关功能中，只有距离分隔的功能可以预测2PA强度的顺序，与实验结果和RI-CC2结果一致。即使范围分隔的功能可以预测正确的相对趋势，对于所考虑的分子，2PA强度的绝对值被低估了2–6。根据导出的耦合簇波函数的2PA强度的广义少数状态模型表达式进行的深入分析表明，这些函数的问题可能与被低估的激发态偶极矩有关，并且在较小程度上，高估了激发能。半本地和混合功能显示出较少的可预测错误，并且2PA强度的变化与参考结果不一致。半局部函数和混合函数显示的平均误差小于距离分隔函数的平均误差，但是我们的分析表明，这是由于激励能量和跃迁偶极矩之间的偶然误差抵消所致。