Computational Chemistry, Institute of Chemistry, University of Potsdam , D-14476 Potsdam, Germany.
Acc Chem Res. 2014 Nov 18;47(11):3284-91. doi: 10.1021/ar500021t. Epub 2014 Apr 30.
Density functional theory (DFT) and its time-dependent extension (TD-DFT) are powerful tools enabling the theoretical prediction of the ground- and excited-state properties of organic electronic materials with reasonable accuracy at affordable computational costs. Due to their excellent accuracy-to-numerical-costs ratio, semilocal and global hybrid functionals such as B3LYP have become the workhorse for geometry optimizations and the prediction of vibrational spectra in modern theoretical organic chemistry. Despite the overwhelming success of these out-of-the-box functionals for such applications, the computational treatment of electronic and structural properties that are of particular interest in organic electronic materials sometimes reveals severe and qualitative failures of such functionals. Important examples include the overestimation of conjugation, torsional barriers, and electronic coupling as well as the underestimation of bond-length alternations or excited-state energies in low-band-gap polymers. In this Account, we highlight how these failures can be traced back to the delocalization error inherent to semilocal and global hybrid functionals, which leads to the spurious delocalization of electron densities and an overestimation of conjugation. The delocalization error for systems and functionals of interest can be quantified by allowing for fractional occupation of the highest occupied molecular orbital. It can be minimized by using long-range corrected hybrid functionals and a nonempirical tuning procedure for the range-separation parameter. We then review the benefits and drawbacks of using tuned long-range corrected hybrid functionals for the description of the ground and excited states of π-conjugated systems. In particular, we show that this approach provides for robust and efficient means of characterizing the electronic couplings in organic mixed-valence systems, for the calculation of accurate torsional barriers at the polymer limit, and for the reliable prediction of the optical absorption spectrum of low-band-gap polymers. We also explain why the use of standard, out-of-the-box range-separation parameters is not recommended for the DFT and/or TD-DFT description of the ground and excited states of extended, pi-conjugated systems. Finally, we highlight a severe drawback of tuned range-separated hybrid functionals by discussing the example of the calculation of bond-length alternation in polyacetylene, which leads us to point out the challenges for future developments in this field.
密度泛函理论（DFT）及其时间相关扩展（TD-DFT）是强大的工具，能够以合理的精度和可承受的计算成本预测有机电子材料的基态和激发态性质。由于其出色的精度与计算成本比，半局部和全局混合泛函（如 B3LYP）已成为现代理论有机化学中几何优化和振动光谱预测的主力。尽管这些现成的泛函在这些应用中取得了压倒性的成功，但对于有机电子材料中特别感兴趣的电子和结构性质的计算处理有时会暴露出这些泛函的严重和定性失败。重要的例子包括对共轭、扭转势垒和电子耦合的高估以及对低带隙聚合物中的键长交替或激发态能量的低估。在本报告中，我们强调了这些失败如何可以追溯到半局部和全局混合泛函固有的离域误差，这导致电子密度的虚假离域和共轭的高估。通过允许最高占据分子轨道的分数占据，可以量化对感兴趣的系统和泛函的离域误差。可以通过使用长程校正混合泛函和非经验的范围分离参数调谐过程来最小化离域误差。然后，我们回顾了使用调谐长程校正混合泛函来描述π共轭系统的基态和激发态的优缺点。特别是，我们表明，这种方法为有机混合价系统中的电子耦合提供了稳健且高效的特征化手段，为聚合物极限下准确扭转势垒的计算以及低带隙聚合物的可靠光学吸收光谱的预测提供了手段。我们还解释了为什么不建议使用标准的、现成的范围分离参数来进行扩展的π共轭系统的 DFT 和/或 TD-DFT 描述。最后，通过讨论聚乙炔中键长交替的计算示例，我们强调了调谐范围分离混合泛函的一个严重缺点，这使我们指出了该领域未来发展的挑战。
