分子晶体超原胞声子计算中计算捷径的评价：萘的指导性例子
Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene
摘要：
声子对有机半导体材料的各种性质有着至关重要的影响。例如，电荷和热的传输依赖于低频声子，而对于其他性质，如自由能，特别是高频声子计数。对于所有这些量，人们需要知道整个声子能带结构，当使用色散校正密度泛函理论（DFT）等方法时，对于更复杂系统，其模拟变得非常昂贵。因此，在目前的贡献，我们评估的性能更近似的方法，包括密度泛函紧束缚（DFTB）和不同的复杂性和复杂性的力场（FF）池。除了仅仅比较声子能带结构，我们还严格评估到什么程度的导出量，如温度依赖的热容，均方热位移和温度依赖的自由能的影响的缺陷的声子带的描述。作为一个基准系统，我们选择（氘代）萘，作为唯一的有机半导体材料，迄今为止的实验声子能带结构在文献中。总的来说，最好的性能之间的近似方法是观察系统具体参数化的第二代力场。有趣的是，在低频区域，具有相当简单的键相互作用模型的力场（如通用琥珀力场）表现得相当好。就测试的DFTB参数化而言，我们得到一个显着低估的单位晶胞体积导致在一个显着高估的声子能量在低频区域。这不能通过依赖DFT计算的单位晶胞来弥补，因为使用该单位晶胞，DFTB声子频率显著低估了实验。
英文摘要：
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 amongst 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.
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