有限温度下核量子效应对表面介导水解离的决定性作用
Decisive role of nuclear quantum effects on surface mediated water dissociation at finite temperature
摘要：
吸附在无机基质上的水分子在一些技术应用中起着重要作用。当吸附物中存在轻原子时，核量子效应（NQE）影响着这些体系的性质。在这项工作中，我们探讨了NQE对水线在台阶Pt（221）表面上解离的影响。通过用范德华修正的密度泛函理论进行从头计算分子动力学模拟，我们注意到，在有限的温度下，完整和解离结构的几个竞争极小值是可访问的，这使得重要的是要评估量子自由能的谐波估计是否足以确定不同状态的相对稳定性。我们进行从头算路径积分分子动力学（PIMD），以计算这些贡献，考虑到构象熵和非谐性在有限的温度下。我们建议，当吸附是弱的，基板上的NQE是可以忽略不计的，PIMD模拟可以通过一个简单的分区的系统，从而在相当大的计算节省。我们计算了NQE对自由能的贡献，包括非谐项。我们发现，它们导致高达20%的量子贡献的离解自由能的谐波估计相比，增加。我们还发现，解离有一个可以忽略不计的贡献隧道，但占主导地位的ZPE，这可以提高三个数量级的速率。最后，我们强调温度和NQE如何间接影响偶极子和电子密度的重新分布，导致功函数的变化高达0.4 eV的静态估计。功函数的变化的定量测定提供了一种可能的方法，以确定实验上的阶梯表面上的水低聚物的最稳定配置。
英文摘要：
Water molecules adsorbed on inorganic substrates play an important role in several technological applications. In the presence of light atoms in adsorbates, nuclear quantum effects (NQE) influence properties of these systems. In this work, we explore the impact of NQE on the dissociation of water wires on stepped Pt(221) surfaces. By performing ab initio molecular dynamics simulations with van der Waals corrected density functional theory, we note that several competing minima for both intact and dissociated structures are accessible at finite temperatures, making it important to assess whether harmonic estimates of the quantum free energy are sufficient to determine the relative stability of the different states. We perform ab initio path integral molecular dynamics (PIMD) in order to calculate these contributions taking into account conformational entropy and anharmonicities at finite temperatures. We propose that when when adsorption is weak and NQE on the substrate are negligible, PIMD simulations can be performed through a simple partition of the system, resulting in considerable computational savings. We calculate the contribution of NQE to the free energies, including anharmonic terms. We find that they result in an increase of up to 20% of the quantum contribution to the dissociation free energy compared to harmonic estimates. We also find that the dissociation has a negligible contribution from tunneling, but is dominated by ZPE, which can enhance the rate by three orders of magnitude. Finally we highlight how both temperature and NQE indirectly impact dipoles and the redistribution of electron density, causing work function to changes of up to 0.4 eV with respect to static estimates. This quantitative determination of the change in work function provides a possible approach to determine experimentally the most stable configurations of water oligomers on the stepped surfaces.
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