Critical Importance of van der Waals Stabilization in Strongly Chemically Bonded Surfaces: Cu(110):O.

We provide strong evidence that different reconstructed phases of the oxidized Cu(110) surface are stabilized by the van der Waals (vdW) interactions. These covalently bonded reconstructed surfaces feature templates that are an integral part of the surfaces and are bonded on the bare metal surface by a combination of chemical and physical bonding. The vdW stabilization in this class of systems affects predominantly the intertemplate Cu-O interactions in structures sparsely populated by these templates. The conventional dispersionless density functional theory (DFT) methods fail to model such systems. We find a failure to describe the thermodynamics of the different phases that are formed at different oxygen exposures and spurious minima on the potential energy surface of a diffusing surface adatom. To overcome these issues, we employ a range of different DFT methods that account for the missing vdW correlations. Surprisingly, despite vast conceptual differences in the different formulations of these methods, they yield physically identical results for the Cu(110):O surface phases, provided the massive screening effects in the metal are taken into account. Contrary, the vibrational contribution does not consistently stabilize the experimentally observed surface structures. The van der Waals surface stabilization, so far deemed to play only a minor role in hard-bonded surfaces, is suggested to be a more general key feature for this and other related surfaces.