  Within a 2-D scattering model, we investigate the vibrational relaxation of
an idealized molecule colliding with a metal surface. Two perturbative
nonadiabatic dynamics schemes are compared: $(i)$ electronic friction (EF) and
$(ii)$ classical master equations (CME). In addition, we also study a third
approach, $(iii)$ a broadened classical master equation (BCME) that
interpolates between approaches $(i)$ and $(ii)$. Two conclusions emerge.
First, even though we do not have exact data to compare against, we find there
is strong evidence suggesting that EF results may be spurious for scattering
problems with more than one nuclear dimension. Second, we find that there is an
optimal molecule-metal coupling that maximizes vibrational relaxation rates by
inducing large nonadiabatic interactions.
