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首页> 外文期刊>Journal of chemical theory and computation: JCTC >Assessment of Approximate Methods for Anharmonic Free Energies
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Assessment of Approximate Methods for Anharmonic Free Energies

机译：跨杆自由能量评估近似方法

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Quantitative evaluation of the thermodynamic properties of materials—most notably their stability, as measured by the free energy—must take into account the role of thermal and zero-point energy fluctuations. While these effects can easily be estimated within a harmonic approximation, corrections arising from the anharmonic nature of the interatomic potential are often crucial and require computationally costly path integral simulations to obtain results that are essentially exact for a given potential. Consequently, different approximate frameworks for computing affordable estimates of the anharmonic free energies have been developed over the years. Understanding which of the approximations involved are justified for a given system, and therefore choosing the most suitable method, is complicated by the lack of comparative benchmarks. To facilitate this choice we assess the accuracy and efficiency of some of the most commonly used approximate methods: the independent mode framework, the vibrational self-consistent field, and self-consistent phonons. We compare the anharmonic correction to the Helmholtz free energy against reference path integral calculations. These benchmarks are performed for a diverse set of systems, ranging from simple weakly anharmonic solids to flexible molecular crystals with freely rotating units. The results suggest that, for simple solids such as allotropes of carbon, these methods yield results that are in excellent agreement with the reference calculations, at a considerably lower computational cost. For more complex molecular systems such as polymorphs of ice and paracetamol the methods do not consistently provide a reliable approximation of the anharmonic correction. Despite substantial cancellation of errors when comparing the stability of different phases, we do not observe a systematic improvement over the harmonic approximation even for relative free energies. We conclude that, at least for the classes of materials considered here, efforts toward obtaining computationally feasible anharmonic free energies should therefore be directed toward reducing the expense of path integral methods.
机译：通过自由能量测量的材料的热力学性能的定量评估 - 最符合其稳定性 - 必须考虑到热和零点能量波动的作用。虽然可以在谐波近似内容易地估计这些效果，但是由内部潜力的anharmonic性质引起的校正通常是至关重要的，并且需要计算昂贵的路径积分模拟，以获得基本上精确的潜力的结果。因此，多年来，已经开发了用于计算无摇臂态自由能量的价格实惠估计的不同近似框架。了解所涉及的近似值对于给定的系统是合理的，因此选择最合适的方法，因缺乏比较基准而变得复杂。为了促进这种选择，我们评估一些最常用的近似方法的准确性和效率：独立模式框架，振动自我一致的字段和自我一致的声子。我们将Anharmonic校正与亥姆霍兹自由能量进行比较，反对参考路径积分计算。这些基准是针对多种系统进行的，从简单的弱Anharconic固体到柔性分子晶体，与自由旋转单元。结果表明，对于简单的固体，例如碳的同种异体，这些方法的结果与参考计算的优异一致性，以相当较低的计算成本。对于更复杂的分子系统，例如冰和乙酰氨基酚的多晶型物，该方法不始终如一地提供无谐波校正的可靠逼近。尽管在比较不同阶段的稳定性时，尽管在比较了不同阶段的稳定性时，即使对于相对的自由能，我们也不会观察到谐波近似的系统改善。我们得出结论，至少对于这里考虑的材料的类别，因此应该针对降低路径整体方法的牺牲来指导获得计算可行的anharmonic自由能的努力。

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