Phonons crucially impact a variety of properties of organic semiconductor materials. For instance, charge- and heat transport depend on low-frequency phonons, while for other properties, such as the free energy, especially high-frequency phonons count. For all these quantities one needs to know the entire phonon band structure, whose simulation becomes exceedingly expensive for more complex systems when using methods like dispersion-corrected density functional theory (DFT). Therefore, in the present contribution we evaluate the performance of more approximate methodologies, including density functional tight binding (DFTB) and a pool of force fields (FF) of varying complexity and sophistication. Beyond merely comparing phonon band structures, we also critically evaluate to what extent derived quantities, like temperature-dependent heat capacities, mean squared thermal displacements, and temperature-dependent free energies are impacted by shortcomings in the description of the phonon bands. As a benchmark system, we choose (deuterated) naphthalene, as the only organic semiconductor material for which to date experimental phonon band structures are available in the literature. Overall, the best performance among the approximate methodologies is observed for a system-specifically parametrized second-generation force field. Interestingly, in the low-frequency regime also force fields with a rather simplistic model for the bonding interactions (like the General Amber Force Field) perform rather well. As far as the tested DFTB parametrization is concerned, we obtain a significant underestimation of the unit-cell volume resulting in a pronounced overestimation of the phonon energies in the low-frequency region. This cannot be mended by relying on the DFT-calculated unit cell, since with this unit cell the DFTB phonon frequencies significantly underestimate the experiments.

中文翻译：

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在基于超级单元的分子晶体声子计算中评估计算捷径：萘的教学案例。

声子至关重要地影响有机半导体材料的各种特性。例如，电荷和热的传输取决于低频声子，而对于其他性质，例如自由能，尤其是高频声子的数量。对于所有这些量，都需要了解整个声子带结构，当使用诸如色散校正的密度泛函理论（DFT）之类的方法时，其模拟对于更复杂的系统而言变得极其昂贵。因此，在本文中，我们评估了更近似方法的性能，包括密度功能紧密结合（DFTB）和复杂性和复杂程度各异的力场（FF）池。除了仅比较声子能带结构之外，我们还严格评估了导出量的大小，例如与温度相关的热容，均方根热位移和取决于温度的自由能受声子带描述中的缺点影响。作为基准系统，我们选择（氘）萘作为迄今为止文献中可获得的实验声子能带结构的唯一有机半导体材料。总体而言，对于系统特定的参数化第二代力场，可以观察到近似方法中的最佳性能。有趣的是，在低频状态下，具有键合相互作用的简化模型的力场（如通用琥珀色力场）也表现良好。至于经过测试的DFTB参数化，我们获得了单位单元体积的明显低估，从而导致低频区域中声子能量的明显高估。依靠DFT计算得出的晶胞无法解决这一问题，因为使用该晶胞，DFTB声子频率会大大低估实验。