  Molecular adsorption at organic/metal interfaces depends on a range of
mechanisms: covalent bonds, charge transfer, Pauli repulsion and van der Waals
(vdW) interactions shape the potential energy surface (PES), making it key to
understanding organic/metal interfaces. Describing such interfaces with density
functional theory requires carefully selecting the exchange correlation (XC)
functional and vdW correction scheme. To explore the reproducibility of the PES
with respect to the choice of method, we present a benchmark of common local,
semi-local and non-local XC functionals in combination with various vdW
corrections. We benchmark these methods using perylenetetracarboxylic
dianhydride (PTCDA) on Ag(111), one of the most frequently studied
organic/metal interfaces. For each method, we determine the PES using a
Gaussian process regression algorithm, which requires only about 50 DFT
calculations as input. This allows a detailed analysis of the PESs' features,
such as the positions and energies of minima and saddle points. Comparing the
results from different combinations of XC functionals and vdW corrections
enables us to identify trends and differences between the approaches. PESs for
different computation methods are in qualitative agreement, but also displaying
significant quantitative differences. In particular, lateral positions of
adsorption geometries agree well with experiment, while adsorption heights,
energies and barriers show larger discrepancies.
