Adsorption of hydrogen atoms on a single graphite sheet ͑graphene͒ has been investigated by first-principles electronic structure means, employing plane-wave based periodic density functional theory. A 5 ϫ 5 surface unit cell has been employed to study single and multiple adsorptions of H atoms. Binding and barrier energies for sequential sticking have been computed for a number of configurations involving adsorption on top of carbon atoms. We find that binding energies per atom range from ϳ0.8 to ϳ1.9 eV, with barriers to sticking in the range 0.0-0.15 eV. In addition, depending on the number and location of adsorbed hydrogen atoms, we find that magnetic structures may form in which spin density localizes on a ͱ 3 ϫ ͱ 3R30°sublattice and that binding ͑barrier͒ energies for sequential adsorption increase ͑decrease͒ linearly with the site-integrated magnetization. These results can be rationalized with the help of the valence-bond resonance theory of planar conjugated systems and suggest that preferential sticking due to barrierless adsorption is limited to formation of hydrogen pairs.
We report on the local atomic and electronic structures of a nitrogen-doped graphite surface by scanning tunneling microscopy, scanning tunneling spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. The nitrogen-doped graphite was prepared by nitrogen ion bombardment followed by thermal annealing. Two types of nitrogen species were identified at the atomic level: pyridinic-N (N bonded to two C nearest neighbors) and graphitic-N (N bonded to three C nearest neighbors). Distinct electronic states of localized π states were found to appear in the occupied and unoccupied regions near the Fermi level at the carbon atoms around pyridinic-N and graphitic-N species, respectively. The origin of these states is discussed based on experimental results and theoretical simulations.
Three-dimensional potential energy surfaces (PESs) have been computed, and numerically fitted, for the two lowest electronic states of the LiH2+ system, which are of importance for the astrophysically relevant LiH++H→Li++H2 and LiH+H+→Li+H2+ exoergic reactions. We extend the recently computed 11 000 multi reference valence bond ab initio energy values [Martinazzo et al., Chem. Phys. 287, 335 (2003)] with 600 multireference configuration interaction calculations with complete active self-consistent field reference functions and a large Li(12s10p4d1f)/H(8s6p3d1f) basis set. We have fitted the full set of energy values with a modified Aguado–Paniagua ansatz that correctly takes into account in this ionic system the important long-range contributions to the potential. Calibration calculations on the three-body potential term and the use of essentially exact results for the two-body contributions allow us to estimate the overall accuracy of the analytic PESs to be within that required for accurate quantum scattering calculations. The above reactions can be treated adiabatically because of the large energy gap separating the two electronic states. The relevant potential energy surfaces have a very different shape. On the one hand, the ground-state PES shows a simple structure, with a downhill route to the products and a shallow well at the C2v geometry which lies 0.286 eV below the Li++H2 asymptote. On the other hand, the first excited state is characterized by one deep, dipole-charge well which lies 1.315 eV below the LiH+H+ asymptote, one charge-induced dipole well 0.586 eV below the Li+H2+ asymptote, and a saddle point between them which lies 0.227 eV below the LiH+H+ asymptote. A conical intersection with the second excited state has been found but not yet studied in detail, since we deemed it to be of no direct relevance for the above reactions.
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