Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany.
Department of Chemistry, University of California Davis, One Shields Ave., Davis, California 95616, USA.
J Chem Phys. 2018 Mar 14;148(10):102320. doi: 10.1063/1.5002537.
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 (NQEs) influence the structural stability and the dynamical properties of these systems. In this work, we explore the impact of NQEs 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 thus perform ab initio path integral molecular dynamics (PIMD) in order to calculate these contributions taking into account the conformational entropy and anharmonicities at finite temperatures. We propose that when adsorption is weak and NQEs on the substrate are negligible, PIMD simulations can be performed through a simple partition of the system, resulting in considerable computational savings. We then calculate the full contribution of NQEs to the free energies, including also anharmonic terms. We find that they result in an increase of up to 20% of the quantum contribution to the dissociation free energy compared with the harmonic estimates. We also find that the dissociation process has a negligible contribution from tunneling but is dominated by zero point energies, which can enhance the rate of dissociation by three orders of magnitude. Finally we highlight how both temperature and NQEs indirectly impact dipoles and the redistribution of electron density, causing work function changes of up to 0.4 eV with respect to static estimates. This quantitative determination of the change in the work function provides a possible approach to determine experimentally the most stable configurations of water oligomers on the stepped surfaces.
水分子在无机基底上的吸附在许多技术应用中起着重要作用。在吸附物中有轻原子存在的情况下，核量子效应（NQEs）会影响这些体系的结构稳定性和动力学性质。在这项工作中，我们研究了 NQEs 对在阶梯状 Pt(221)表面上水线解离的影响。通过使用范德华修正密度泛函理论进行从头算分子动力学模拟，我们注意到，在有限温度下，完整和解离结构的几个竞争极小值都是可达到的，因此有必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。因此，我们进行了从头算路径积分分子动力学（PIMD），以计算这些贡献，同时考虑有限温度下的构象熵和非谐性。我们提出，当吸附较弱且基底上的 NQEs 可以忽略不计时，通过简单地对系统进行分区，就可以进行 PIMD 模拟，从而大大节省计算成本。然后，我们计算了 NQEs 对自由能的全部贡献，包括非谐项。我们发现，与谐波估计相比，它们使解离自由能的量子贡献增加了高达 20%。我们还发现，解离过程中隧道效应的贡献可以忽略不计，但零点能起主导作用，这可以使解离速率提高三个数量级。最后，我们强调了温度和 NQEs 如何间接影响偶极子和电子密度的重新分布，导致功函数相对于静态估计发生高达 0.4 eV 的变化。这种对功函数变化的定量确定为实验上确定阶梯状表面上水齐聚物的最稳定构型提供了一种可能的方法。
