Using sound physical principles we modify the DFT-D2 atom pairwise semiempirical dispersion correction to density functional theory to work for metallic systems and in particular self-assembled monolayers of thiols on gold surfaces. We test our approximation for two functionals PBE-D and revPBE-D for lattice parameters and cohesive energies for Ni, Pd, Pt, Cu, Ag, and Au, adsorption energies of CO on (111) surfaces of Pd, Pt, Cu, Ag, and Au, and adsorption energy of benzene on Ag(111) and Au(111). Agreement with experimental data is substantially improved. We apply the method to self-assembled monolayers of alkanethiols on Au(111) and find reasonable agreement for PBE-D and revPBE-D for both physisorption of n-alkanethiols as well as dissociative chemisorption of dimethyl disulfide as an Au-adatom-dithiolate complex. By modifying the coefficient for Au, we obtain quantitative agreement for physisorption and chemisorption for both PBE-D and revPBE-D using the same set of parameters. Our results confirm that inclusion of dispersion forces is crucial for any quantitative analysis of the thiol and thiolate bonds to the gold surface using quantum chemical calculations. 1. Introduction Density functional theory (DFT) is the method of choice for first-principles calculations in condensed matter systems and has contributed greatly to our understanding of metallic systems such as heterogeneous catalysis of ammonia synthesis [1]. However, conventional DFT functionals do not take into account van der Waals interactions, that is, London dispersion. These interactions are crucial for many systems such as interlayer bonding in graphite [2] and biological systems. Research in the last decade has led to dispersion being included in DFT in many ways [3]. Some methods that have the correct asymptotic behavior are nonlocal dispersion-density functional (vdW-DF) [4, 5] and semiempirical atom pairwise dispersion [6–9]. Some highly parameterized meta-GGA functionals also include short-range dispersion effects, like the M06 family of functionals [10], but do not have the correct long-range asymptotic behavior [3]. The vdW-DF functional takes into account electronic effects such as electron transfer from first-principles. Its accuracy for normal thermochemistry is however not well established yet. It is furthermore not well defined for spin-polarized systems, such as Fe, Ni, Co, and their alloys. Dispersion effects included via semiempirical atom pairwise interactions using the DFT-D2 or DFT-D3 methods by Grimme et al. have been shown to give quite accurate thermochemistry for
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