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.