Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene
Evaluating Computational Shortcuts in Supercell-Based Phonon Calculations of Molecular Crystals: The Instructive Case of Naphthalene
复制标题
DOI：
10.1021/acs.jctc.0c00119
复制
发表时间：
2020-04-14
影响因子：
5.5
通讯作者：
Zojer, Egbert
中科院分区：
文献类型：
--
作者：
Kamencek, Tomas;Wieser, Sandro;Zojer, Egbert
声子对有机半导体材料的多种性能产生至关重要的影响。例如，电荷和热传输取决于低频声子，而对于其他特性，例如自由能，尤其是高频声子计数。对于所有这些量，我们需要了解整个声子能带结构，当使用色散校正密度泛函理论 (DFT) 等方法时，对于更复杂的系统来说，声子能带结构的模拟变得极其昂贵。因此，在当前的贡献中，我们评估了更近似方法的性能，包括密度泛函紧束缚（DFTB）和不同复杂度和复杂程度的力场（FF）池。除了比较声子能带结构之外，我们还批判性地评估了声子能带描述中的缺陷对派生量（例如与温度相关的热容、均方热位移和与温度相关的自由能）的影响程度。作为基准系统，我们选择（氘代）萘，作为迄今为止文献中可获得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，观察到了近似方法中的最佳性能。有趣的是，在低频状态下，具有相当简单的键合相互作用模型的力场（如通用琥珀力场）也表现得相当好。就测试的 DFTB 参数化而言，我们明显低估了晶胞体积，导致低频区域的声子能量明显高估。这不能通过依赖 DFT 计算的晶胞来弥补，因为使用该晶胞时 DFTB 声子频率明显低估了实验。
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.