Iyer Gopal R, Rubenstein Brenda M
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States.
J Phys Chem A. 2022 Jul 21;126(28):4636-4646. doi: 10.1021/acs.jpca.2c01957. Epub 2022 Jul 12.
The accurate prediction of reaction mechanisms in heterogeneous (surface) catalysis is one of the central challenges in computational chemistry. Quantum Monte Carlo methods─Diffusion Monte Carlo (DMC) in particular─are being recognized as higher-accuracy, albeit more computationally expensive, alternatives to Density Functional Theory (DFT) for energy predictions of catalytic systems. A major computational bottleneck in the broader adoption of DMC for catalysis is the need to perform finite-size extrapolations by simulating increasingly large periodic cells (supercells) to eliminate many-body finite-size effects and obtain energies in the thermodynamic limit. Here, we show that it is possible to significantly reduce this computational cost by leveraging the cancellation of many-body finite-size errors that accompanies the evaluation of energy differences when calculating quantities like adsorption (binding) energies and mapping potential energy surfaces. We analyze the cancellation and convergence of many-body finite-size errors in two well-known adsorbate/slab systems, HO/LiH(001) and CO/Pt(111). Based on this analysis, we identify strategies for obtaining binding energies in the thermodynamic limit that optimally utilize error cancellation to balance accuracy and computational efficiency. Using one such strategy, we then predict the correct order of adsorption site preference on CO/Pt(111), a challenging problem for a wide range of density functionals. Our accurate and inexpensive DMC calculations are found to unambiguously recover the top > bridge > hollow site order, in agreement with experimental observations. We proceed to use this DMC method to map the potential energy surface of CO hopping between Pt(111) adsorption sites. This reveals the existence of an L-shaped top-bridge-hollow diffusion trajectory characterized by energy barriers that provide an additional kinetic justification for experimental observations of CO/Pt(111) adsorption. Overall, this work demonstrates that it is routinely possible to achieve order-of-magnitude speedups and memory savings in DMC calculations by taking advantage of error cancellation in the calculation of energy differences that are ubiquitous in heterogeneous catalysis and surface chemistry more broadly.
准确预测多相（表面）催化中的反应机理是计算化学的核心挑战之一。量子蒙特卡罗方法，特别是扩散蒙特卡罗（DMC）方法，尽管计算成本更高，但被认为是用于催化系统能量预测的比密度泛函理论（DFT）精度更高的替代方法。在更广泛地将DMC应用于催化领域时，一个主要的计算瓶颈是需要通过模拟越来越大的周期性晶胞（超胞）来进行有限尺寸外推，以消除多体有限尺寸效应并在热力学极限下获得能量。在此，我们表明，在计算吸附（结合）能和绘制势能面等物理量时，利用计算能量差时多体有限尺寸误差的抵消，可以显著降低这种计算成本。我们分析了两个著名的吸附质/平板系统HO/LiH(001)和CO/Pt(111)中多体有限尺寸误差的抵消和收敛情况。基于此分析，我们确定了在热力学极限下获得结合能的策略，这些策略能最佳地利用误差抵消来平衡精度和计算效率。使用其中一种策略，我们随后预测了CO在Pt(111)上吸附位点偏好的正确顺序，这对于广泛的密度泛函来说是一个具有挑战性的问题。我们精确且低成本的DMC计算明确地恢复了顶位>桥位>空心位的顺序，与实验观察结果一致。我们接着使用这种DMC方法绘制了CO在Pt(111)吸附位点之间跳跃的势能面。这揭示了存在一条L形的顶位 - 桥位 - 空心位扩散轨迹，其特征是具有能垒，这为CO/Pt(111)吸附的实验观察提供了额外的动力学依据。总体而言，这项工作表明，通过利用多相催化以及更广泛的表面化学中普遍存在的能量差计算中的误差抵消，在DMC计算中常规地实现数量级的加速和内存节省是可能的。
