We use density-functional theory (DFT) to analyse the interaction of trans-
and cis-porphycene with Cu(111) and their interconversion by intramolecular
H-transfer. This tautomerisation reaction is characterised by small values for
the reaction energy and barrier, on the order of ∼0.1 eV, where the trans
configuration is thermodynamically more stable upon adsorption according to
the experiments [J. N. Ladenthin et al., ACS Nano 9, 7287–7295 (2015)]. To
gain even a qualitatively correct description of this reaction at the DFT
level, an accurate treatment of dispersion interactions and a careful choice
of the exchange contribution are required in order to predict the subtle
energetics. Analysis of the electronic structure shows that adsorption is
contributed by a van der Waals (vdW) interaction, mainly responsible for
stabilising the polyaromatic fragments, and by a significant charge
redistribution localised between Cu and the unsaturated N atoms of the
molecule central cavity. We find that different vdW functionals can produce
qualitatively different electronic structures, while yielding small trans vs.
cis energy differences. Unlike other functionals surveyed here, vdW-DF with
PBE exchange satisfactorily reproduces not only the experimental energetics
but also the scanning tunneling microscopy images. This gives us confidence
that this functional achieves a reliable balance between the two mechanisms
contributing to the adsorption of porphycene