Conventional density functional theory (DFT) is the common choice of researchers. However, recently the density functional tight binding (DFTB) methods and composite B97-3c methods have been used as a faster and accurate DFT alternative. Moreover, the B97-3c based time-dependent DFT, simplified Tamm–Dancoff (sTDA) and simplified TDDFT (sTDDFT) calculations have expanded the domain of state of art excited state calculations. Here, the recently developed organic nanosized emitters 3-(9H-[3,9′-bicarbazol]-9-yl)-9H-xanthen-9-one (CCX-I) and 3-(9′H-[9,3′:6′,9″-tercarbazol]-9′-yl)-9H-xanthen-9-one (CCX-II) have been used as samples to study the performance of the above mentioned methods at the ground and excited levels in gas and solvent conditions, both with and without the dispersion corrections respectively. Bond lengths calculated by DFTB and B97-3c approaches are in excellent agreement with the DFT results. Root mean square deviation (RMSD) values suggest that the DFTB approach provides better band gap values than B97-3c. But, B97-3c results in better torsion angles. Optical properties are found to be closest to the DFT values when calculated by time-dependent DFTB (TD-DFTB) approach with the lowest RMSDs for excitation energies (0.75 eV) and oscillator strengths (~0.08). The computational speed boosts of the B97-3c based sTDDFT and sTDA approaches are ~ 4 and ~33 times faster than the TDDFT counterpart.

中文翻译：

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通过DFT，SCC-DFTB和B97-3c方法对热激活的延迟荧光纳米发射体（CCX-I和CCX-II）的电子和光学参数进行性能研究

常规密度泛函理论（DFT）是研究人员的常见选择。但是，最近密度功能紧密结合（DFTB）方法和复合B97-3c方法已被用作更快，更准确的DFT替代方法。此外，基于B97-3c的基于时间的DFT，简化的Tamm-Dancoff（sTDA）和简化的TDDFT（sTDDFT）计算已经扩展了最新的激发态计算范围。在此，最近开发的有机纳米发射体3-（9- ħ - [3,9'-bicarbazol] -9-基）-9- ħ -呫吨-9-酮（CCX-I）和3-（9' ħ - [ 9,3'：6'，9''-叔咔唑] -9'-yl）-9 H-xanthen-9-one（CCX-II）已用作样品，分别研究了上述方法在气体和溶剂条件下在地面和激发能级下的性能，并分别进行了色散校正和不进行色散校正。DFTB和B97-3c方法计算的键长与DFT结果非常吻合。均方根偏差（RMSD）值表明，DFTB方法比B97-3c提供更好的带隙值。但是，B97-3c导致更好的扭转角。当通过具有时间依赖性的DFTB（TD-DFTB）方法进行计算时，发现光学特性最接近DFT值，其激发能量（0.75 eV）和振荡器强度（〜0.08）的RMSD最低。基于B97-3c的sTDDFT和sTDA方法的计算速度提升是TDDFT对应方法的约4倍和约33倍。