Böttcher et al. Nanoscale Research Letters 2011, 6:214 http://www.nanoscalereslett.com/content/6/1/214 Page 5 of 7 Figure 4 Series of the ARPES spectra obtained on graphene/Ni(111), H2O/graphene/Ni(111), and NH3/graphene/Ni(111) along the Γ-K direction of the Brillouin zone. The amounts of water and ammonia were estimated as 0.5 of the ML. These data were collected with the photon energy of 75 eV. interaction arising due to two types of forces, namely, dispersion forces and/or classical electrostatic ones. The dispersion interactions are long-range electron correla- tion effects, which are not captured in DFT because of the local character of common functionals. Consequently, DFT often fails to describe physisorption correctly. For a correct and consistent treatment of physisorption inter- action, it is necessary to use high-level wave-function- based post-Hartree-Fock methods like the Møller-Plesset perturbation theory [11] or the coupled-cluster (CC) method [12]. One problem here is that a very accurate treatment, e.g., with the CC method, scales very unfavor- ably with the number of electrons in the system. In gen- eral, this difficulty is avoidable by employing the so- called method of increments, where the correlation energy is written in terms of contributions from localized orbital groups [13]. An alternative approach is an inclu- sion of the dispersion correction to the total energy obtained with standard DFT approximation explicitly by hand with, e.g., DFT-D method, that is atom pair-wise sum over C6R-6 potentials (see, e.g., Ref. [14]). Recently, studies based on first principles for single H2O molecule adsorbed on freestanding graphene were performed by O. Leenaerts et al. [15]. For comparison purpose, the reported interaction energies (Eint) are listed in Table 1 together with the corresponding equili- brium distances (d0). (The VASP-calculations of this study for (3 × 3) supercell yield similar values). One can observe very low interaction energies and no energetic preference regarding the adsorption site or orientation of the adsorbate. We have repeated the calculations tak- ing into account the dispersion correction as proposed by Grimme [16] (DFT-D2 method). The resulting inter- action energies are higher by 4-7 times in magnitude, although still physisorption is predicted coincidently with experimental observations. Consequently, the equi- librium distances between H2O and graphene are signifi- cantly shorter. In addition, DOWN orientation is clearly more preferred in this case as compared to the opposite one (i.e., UP). It can be noted that the obtained results are in reasonable agreement with the recent CCSD(T) data evaluated for the H2O/graphene system. Thus, for further consideration of the systems of interest, the PBE-D2 approximation will be used. Table 1 The interaction energies (Eint) and the equilibrium distances (d0) between H2O and the surface of the freestanding graphene layer as obtained for the six selected geometries at DFT level with standard PBE functional and when including dispersion correction (PBE-D2) Geometry C_DOWN C_UP B_DOWN B_UP T_DOWN T_UP aData taken from Ref. [15]. d0 (Å) 4.02 3.69 4.05 3.70 4.05 3.70 PBEa Eint (meV) 19 20 18 18 19 19 d0 (Å) 2.60 3.07 2.67 3.17 2.64 3.18 PBE-D2 Eint (meV) 139 83 129 77 127 75