Abstract
We report the results of Olympicene radical (C19H11) adsorption characteristics on Cu(111) obtained within the density functional framework with and without the inclusion of self-consistent van der Waals (vdW) interactions to evaluate their effects. Our calculations suggest that the vdW interactions enhance the adsorption energies, and the degree of enhancement strongly depends on the implementation. Among the considered configurations, the highest adsorption energy calculated using PBE is found to be 0.24 eV, while those obtained with the inclusion of vdW interactions are 1.32 eV (rPW86), 1.91 eV (optPBE), 2.29 eV (optB88), and 2.65 eV (optB86b). The high energetic contribution obtained using the vdW interactions correlates with the changes in the adsorption heights (from 3.4 angstrom (PBE and rPW86) to 2.24 angstrom (optB86b)), the change in the structural integrity of Olympicene radical upon adsorption, and the changes in the surface electronic structure. Furthermore, our calculations reveal a net charge transfer of 0.39e(-) (the highest) from the substrate to the molecule upon inclusion of vdW interactions (opt-type functionals), while no charge transfer is found using PBE and rPW86 functionals. Upon adsorption, an interface state at about 0.6 eV below the Fermi level is observed, along with a noticeable change in the position of the d-band center and width only when the vdW interactions (opt-type) are considered. These significant changes can be considered as strong indications of a transition in the nature of bonding from the weak physisorption to strong chemisorption. Our results reveal the unavoidable importance of the self-consistent vdW interactions for an accurate description of adsorption characteristics of organic species on metal substrates, and further provide a minimal list of characteristics to be considered for distinguishing the weak physisorption from strong chemisorption.