We present a comparison of exchange-only interaction energies obtained using several standard exchange functionals in the generalized gradient approximation to Hartree-Fock results for interacting molecules. We observe that functionals with an enhancement factor using a 2/5 power dependence on the gradient of the density for large density gradients offer consistently better agreement with Hartree-Fock calculations than that of alternative functionals. We revisit the functional offering the closest agreement and recalculate it to include its exact large gradient dependence.

M olecular hydrogen (H 2 ) sorbents are appealing materials for storing hydrogen fuel onboard vehicles. The uptake and release of H 2 fuel in the sorbent materials can be fast and require less heat transfer. To use the H 2 sorbents at near ambient conditions, the binding energy of H 2 in these materials must be within certain range (e.g., 20Ϫ40 kJ/mol). 1,2 Theoretical studies predicted 3,4 that Kubaslike interactions between transition metal (TM) centers and coordinated H 2 could fall within this desirable energy range. Such predictions are consistent with recent experimental studies by using metalϪorganic frameworks (MOFs) with under-coordinated TM. 5Ϫ8 Attempts to anchor TM directly on carbon nanostructures, however, have not yet been successful. Recently, Hamaed et al. used organometallic precursor to successfully graft Ti onto the inner surface of mesoporous silica. 9 Though this work demonstrated the feasibility of individually dispersing Ti and the capability of binding multi-H 2 by dispersed Ti, mesoporous silica has a relatively small surface-to-volume ratio and may be too heavy for practical hydrogen storage. So far, no practical H 2 sorbent is available. Finding the right material for onboard storage is still a grand challenge. Concerning TM-based organometallic sorbents, several conditions are required at the same time: First, the substrate materials possess high surface-to-volume ratio and are lightweight. Second, the TM atoms are undercoordinated and well-exposed to accommodate multi-H 2 . Third, these unsaturated TM atoms, despite their high chemical reactivity, 10 do not form clusters. These require that the anchoring bonds between the TM atoms and the substrate are strong and the TM coverage is also optimized. Along the line of strengthening the anchoring bonds, several strategies have been suggested, such as functionalizing organic molecules, 11 employing defect sites in carbon materials, 12,13 and directly integrating metal atoms into the skeleton. 14,15 Alternatively, graphene oxide (GO) can be a potential substrate to covalently anchor TM atoms by simultaneously satisfying all these three conditions. GO has large surface-to-volume ratio and is intrinsically lightweight (condition 1). GO possesses ample O sites on the surfaces. Oxygen is the key in anchoring under-coordinated Ti (condition 2) and enhancing the TMϪ substrate binding (condition 3), as having been experimentally demonstrated on mesoporous silica. 9 Although GO has been routinely synthesized and extensively studied, 16Ϫ24 currently its precise atomic structures are still under intense investigation. In fact, the O content of GO can vary greatly, depending

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Detailed experimental data for physisorption potential-energy curves of H2 on low-indexed faces of Cu challenge theory. Recently, density-functional theory has been developed to also account for nonlocal correlation effects, including van der Waals forces. We show that one functional, denoted vdW-DF2, gives a potential-energy curve promisingly close to the experiment-derived physisorptionenergy curve. The comparison also gives indications for further improvements of the functionals.

Nanometer-sized columns of condensed water molecules are created by an atomic-resolution force microscope operated in ambient conditions. Unusual stepwise decrease of the force gradient associated with the thin water bridge in the tip-substrate gap is observed during its stretch, exhibiting regularity in step heights (≈ 0.5 N/m) and plateau lengths (≈ 1 nm). Such "quantized" elasticity is indicative of the atomic-scale stick-slip at the tip-water interface. A thermodynamic-instabilityinduced rupture of the water meniscus (5-nm long and 2.6-nm wide) is also found. This work opens a high-resolution study of the structure and the interface dynamics of a nanometric aqueous column.PACS numbers: 07.79. 07.79.Lh, 47.17.+e, 62.10.+s Water is one of the most important substances of life and has been studied extensively for hundreds of years. Nonetheless, it is still quite a unique matter that keeps surprising us and exhibits peculiarities, in particular, when confined in a nanometric configuration. For example, water and simple organic liquids exhibit solid-like orderedness in molecularly thin films [1,2,3]. Water molecules inside hydrophobic nanotubes manifest phases of ice that are not found under bulk conditions [4]. However, since bulk water possesses only short-range order [5] and water molecules move incessantly, it is usually difficult to experimentally investigate novel features of confined water structures other than thin films.In this Letter, we have employed an atomic-resolution noncontact atomic force microscope (AFM) in air [6] and achieved the spontaneous formation of a nanometric liquid column consisting of thousands of water molecules. We also have performed the sensitive measurement of the elastic property (or the force gradient) of the thin water column during its mechanical stretch. We have thereby demonstrated several novel phenomena: (i) the unusual stepwise decrease of the force gradient, associated with the atomic-scale stick-slip on the AFM-tip surface, (ii) the abrupt rupture of the thin water meniscus due to the thermodynamic instability of the liquid-vapor interface, and (iii) the manipulation of the thin aqueous column by repeated stretch-relaxation cycles, revealing the atomicscale contact angle hysteresis.Water molecules in ambient conditions produce a nanoscale water meniscus between a hydrophilic Si tip and a hydrophilic mica substrate, when capillary condensation occurs as the stiff AFM tip approaches the substrate within a nanometric distance (Fig. 1). Once the thin aqueous column is formed, it is stretched vertically upward by subsequent retraction of the tip. As the molecular water bridge of sub-zeptoliter (zepto = 10 −21 ) volume is elongated thereby, the force gradient associated with the elasticity of the system is measured by an ex- * Corresponding author: whjhe@snu.ac.kr tremely small amplitude-modulation operation of AFM [7,8]. Figure 1 presents the schematics of a home-built AFM setup used for formation of a nanometric water column by capillary condensation as well as for si...