  We employ dispersion-corrected density-functional theory to study the
adsorption of tetrapyrrole 2H-porphine (2H-P) at Cu(111) and Ag(111). Various
contributions to adsorbate-substrate and adsorbate-adsorbate interactions are
systematically extracted to analyze the self-assembly behavior of this basic
building block to porphyrin-based metal-organic nanostructures. This analysis
reveals a surprising importance of substrate-mediated van der Waals
interactions between 2H-P molecules, in contrast to negligible direct
dispersive interactions. The resulting net repulsive interactions rationalize
the experimentally observed tendency for single molecule adsorption.
