Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany.
J Chem Phys. 2017 Aug 21;147(7):074112. doi: 10.1063/1.4985886.
Anisotropic displacement parameters (ADPs) are commonly used in crystallography, chemistry, and related fields to describe and quantify thermal motion of atoms. Within the very recent years, these ADPs have become predictable by lattice dynamics in combination with first-principles theory. Here, we study four very different molecular crystals, namely, urea, bromomalonic aldehyde, pentachloropyridine, and naphthalene, by first-principles theory to assess the quality of ADPs calculated in the quasi-harmonic approximation. In addition, we predict both the thermal expansion and thermal motion within the quasi-harmonic approximation and compare the predictions with the experimental data. Very reliable ADPs are calculated within the quasi-harmonic approximation for all four cases up to at least 200 K, and they turn out to be in better agreement with the experiment than those calculated within the harmonic approximation. In one particular case, ADPs can even reliably be predicted up to room temperature. Our results also hint at the importance of normal-mode anharmonicity in the calculation of ADPs.
各向异性位移参数（ADPs）在晶体学、化学和相关领域中被广泛用于描述和量化原子的热运动。在最近几年，这些 ADP 已经可以通过晶格动力学与第一性原理理论相结合来进行预测。在这里，我们通过第一性原理理论研究了四种非常不同的分子晶体，即尿素、溴代丙二醛、五氯吡啶和萘，以评估在准谐近似下计算的 ADP 的质量。此外，我们还在准谐近似下预测了热膨胀和热运动，并将预测结果与实验数据进行了比较。在所有四种情况下，准谐近似下的 ADP 计算都非常可靠，至少可以达到 200 K，并且它们与实验的一致性比在简谐近似下计算的 ADP 更好。在一个特殊情况下，甚至可以可靠地预测到室温下的 ADP。我们的结果还暗示了在 ADP 计算中，正则模非谐性的重要性。
