Disordered materials make up a large proportion of condensed phase systems, but the difficulties in describing their structures and molecular dynamics limit their potential applications. Disordered crystalline systems, also known as plastic crystals, offer a unique perspective into these factors because the systems retain a degree of crystallinity, reducing the degrees of freedom that must be explored when interpreting the results. However, while disordered crystals do diffract X-rays, it is difficult to fully resolve meaningful crystalline structures, with the best scenario resulting in lattice parameters. In this study, we use a combination of experimental terahertz time-domain spectroscopy, and theoretical solid-state ab initio density functional theory and molecular dynamics simulations to fully elucidate the structures and associated dynamics of organic molecular solids. The results highlight that this combination provides a complete description of the energetic and mechanistic pathways involved in the formation of disordered crystals, and highlights the importance of low-frequency dynamics in their properties. Finally, with structures fully determined and validated by the experimental results, recent progress into anharmonic calculations, namely the quasi-harmonic approximation method, enables full temperature and pressure-dependent properties to be understood within the framework of the potential energy hyper-surface structure.

中文翻译：

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预测无序晶体结构和相关联的相位的转换机制通过实验和理论方法的组合

无序的材料占冷凝相系统的很大一部分，但是描述它们的结构和分子动力学的困难限制了它们的潜在应用。无序晶体系统（也称为塑料晶体）为这些因素提供了独特的视角，因为这些系统保留一定程度的结晶性，从而降低了解释结果时必须探索的自由度。但是，尽管无序晶体确实会衍射X射线，但很难完全解析出有意义的晶体结构，而最好的情况是产生晶格参数。在这项研究中，我们结合使用了实验性的太赫兹时域光谱和理论上的固态从头算密度泛函理论和分子动力学模拟，以充分阐明有机分子固体的结构和相关动力学。结果表明，这种结合提供了对无序晶体形成过程中涉及的能量和机械途径的完整描述，并强调了低频动力学特性的重要性。最后，在通过实验结果完全确定并验证结构的情况下，非谐计算的最新进展，即准谐波近似方法，使得可以在势能超表面结构的框架内理解与温度和压力有关的所有特性。