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Proton-transfer (PT) between organic complexes is a common and importantbiochemical process. Unfortunately, PT energy barriers are difficult toaccurately predict using density functional theory (DFT); in particular, thegeneralized gradient approximation (GGA) tends to underestimate PT barriers.Moreover, PT typically occurs in environments where dispersion forcescontribute to the cohesion of the system; thus, a suitable exchange-correlationfunctional should accurately describe both dispersion forces and PT barriers.This paper provides benchmark results for the PT barriers of several densityfunctionals including several variants of the van der Waals density functional(vdWDF). The benchmark set comprises small organic molecules with inter- andintra-molecular PT. The results show that replacing GGA correlation with afully non-local vdW-DF correlation increases the PT barriers, making it closerto the quantum chemical reference values. In contrast, including nonlocalcorrelations with the Vydrov-Voorhis (VV) method or dispersion-corrections atthe DFT-D3 or the Tkatchenko-Scheffler (TS) level has barely any impact on thePT barriers. Hybrid functionals also increase and improve the energies and thebest performance is provided by a hybrid version of the consistent-exchange vander Waals density functional vdW-DF-cx. For the formic acid dimer PT system, weanalyzed the GGA exchange and non-local correlation contributions. The analysisshows that the repulsive part of the non-local correlation kernel plays a keyrole in the PT energy barriers predicted with vdW-DF.

Improved proton-transfer barriers with van der Waals density functionals: Role of repulsive non-local correlation