Benchmark First-Principles Calculations of Adsorbate Free Energies

Adsorbate
free energies are fundamental quantities in the microkinetic
modeling of catalytic reactions. In first-principles modeling, finite-temperature
free energies are generally obtained by combining density functional
theory energies with standard approximate models, such as the harmonic
oscillator, the hindered translator, or the two-dimensional ideal
gas. In this work, we calculate accurate free energies directly from
first-principles potential energy surfaces combined with exact quantum
mechanical solutions for the translational energy states to benchmark
the reliability of common approximations. Through a series of case
studies of monatomic adsorbates on metal surfaces, we show that no
one free energy model performs satisfactorily in all cases. Moreover,
even combinations of different approximations sometimes deviate significantly
from the free energies calculated by our first-principles approach.
Using observations from these case studies, we discuss how a full
quantum mechanical approach can be extended to calculate accurate
free energies for arbitrary adsorbate potential energy surfaces at
computational cost similar to standard models.

吸附质自由能是催化反应微动力学建模中的基本量。在基于第一性原理的建模中，有限温度下的自由能通常是通过结合密度泛函理论能量与标准近似模型（如谐振子、受阻平移模型或二维理想气体模型）来获得的。在本研究中，我们通过结合第一性原理势能表面与精确的量子力学解对平移能量状态进行计算，从而直接求得准确的自由能，以此验证常见近似模型的可靠性。通过一系列关于金属表面上单原子吸附质的案例研究，我们表明没有任何一种自由能模型在所有情况下都能令人满意。此外，即使是不同近似模型的组合，有时也会显著偏离我们基于第一性原理方法计算得到的自由能。利用这些案例研究的观察结果，我们讨论了如何将完整的量子力学方法扩展至计算任意吸附质势能表面的准确自由能，而其计算成本与标准模型相当。