Attracted by Long-Range Electron Correlation: Adenine on Graphite

The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.

Figure 1

PES (in eV) for adenine adsorbed on graphite calculated within LDA (a), $\mathrm{GGA}+\mathrm{vdW}$ (b), and GGA (c). The minimum energy adsorption position is indicated in (a).

Figure 2

Electron density changes upon adenine adsorption on graphite calculated within LDA. Isodensity surfaces for electron accumulation/depletion of $\pm 2\times {10}^{-5}{\AA }^{-3}$ are shown in blue(+)/red(-). The molecule position is projected on the graphene sheet for clarity.

Figure 3

Molecular energy levels of gas-phase adenine (top left), the graphite surface projected bulk band structure (top right) and selected energy levels of the adsorbed molecule (top middle) calculated within LDA. The energy alignment has been done with respect to the vacuum potential. The graphite Fermi energy is used as energy zero. Isodensity surfaces (${10}^{-6}{\AA }^{-3}$) of the bonding and antibonding combinations $s$ and $s^{*}$ indicated in the energy level diagram are shown below.

Figure 4

Relative total (▪, black), XC (◆, blue) and kinetic energy (of the Kohn-Sham particles; ▾, red) of adenine adsorbed on graphene calculated as a function of the molecule-surface distance. Solid lines are guides to the eye.