  Equilibration dynamics of hot oxygen atoms following O2 dissociation on
Pd(100) and Pd(111) surfaces are investigated by molecular dynamics simulations
based on a scalable neural network potential enabling first-principles
description of O2 and O interacting with variable Pd supercells. We find that
to accurately describe the equilibration dynamics after dissociation, the
simulation cell length necessarily exceeds twice the maximum distance of
equilibrated oxygen adsorbates. By analyzing hundreds of trajectories with
appropriate initial sampling, the measured distance distribution of
equilibrated atom pairs on Pd(111) is well reproduced. However, our results on
Pd(100) suggest that the ballistic motion of hot atoms predicted previously is
a rare event under ideal conditions, while initial molecular orientation and
surface thermal fluctuation could significantly affect the overall
post-dissociation dynamics. On both surfaces, dissociated oxygen atoms remain
primarily locate their nascent positions and then randomly cross bridge sites
nearby.
