We apply the long-range correction (LC) scheme for exchange functionals of density functional theory to time-dependent density functional theory (TDDFT) and examine its efficiency in dealing with the serious problems of TDDFT, i.e., the underestimations of Rydberg excitation energies, oscillator strengths, and charge-transfer excitation energies. By calculating vertical excitation energies of typical molecules, it was found that LC-TDDFT gives accurate excitation energies, within an error of 0.5 eV, and reasonable oscillator strengths, while TDDFT employing a pure functional provides 1.5 eV lower excitation energies and two orders of magnitude lower oscillator strengths for the Rydberg excitations. It was also found that LC-TDDFT clearly reproduces the correct asymptotic behavior of the charge-transfer excitation energy of ethylene-tetrafluoroethylene dimer for the long intramolecular distance, unlike a conventional far-nucleus asymptotic correction scheme. It is, therefore, presumed that poor TDDFT results for pure functionals may be due to their lack of a long-range orbital-orbital interaction.

The performance of different density functional theory ͑DFT͒ methods was investigated in the calculations of the bond length and the binding energy of the first-low transition metal dimers. The 4s-3d interconfigurational energies and 4s and 3d ionization potentials were also calculated for the first-row transition metal atoms. In general, the hybrid DFT method, B3LYP, yields the bond lengths that are too short compared to the experimental ones. In contrast, the optimized bond lengths by nonhybrid DFT methods such as BOP or PW91 are in good agreement with the experiment. It was also found that nonhybrid DFT methods overestimate the binding energies, because they have a tendency to overstabilize the electron configurations that contain the atomic and molecular orbitals in a higher angular momentum open shell. The hybrid DFT method yields more accurate binding energies, but it estimates rather poor energy gaps between states whose spin multiplicity is quite different.

The interaction of water with oxide surfaces has drawn considerable interest, owing to its application to problems in diverse scientific fields. Atomic-scale insights into water molecules on the oxide surface have long been recognized as essential for a fundamental understanding of the molecular processes occurring there. Here, we report the dissociation of a single water molecule on an ultrathin MgO film using low-temperature scanning tunnelling microscopy. Two types of dissociation pathway--vibrational excitation and electronic excitation--are selectively achieved by means of injecting tunnelling electrons at the single-molecule level, resulting in different dissociated products according to the reaction paths. Our results reveal the advantage of using a MgO film, rather than bulk MgO, as a substrate in chemical reactions.

In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu͑111͒, Ag͑111͒, and Au͑111͒ by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals ͑vdW͒ approach ͓S. Grimme, J. Comput. Chem. 27, 1787 ͑2006͔͒ is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance ͑Z C ͒, and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z C Ն 0.35 nm, the work function changes ͑⌬'s͒ do not depend on the substrate work function ͑ m ͒, indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z C Յ 0.25 nm, ⌬'s vary nearly linearly with m , and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction.

Organic molecular semiconductors are solution processable, enabling the growth of large-area single-crystal semiconductors. Improving the performance of organic semiconductor devices by increasing the charge mobility is an ongoing quest, which calls for novel molecular and material design, and improved processing conditions. Here we show a method to increase the charge mobility in organic single-crystal field-effect transistors, by taking advantage of the inherent softness of organic semiconductors. We compress the crystal lattice uniaxially by bending the flexible devices, leading to an improved charge transport. The mobility increases from 9.7 to 16.5 cm2 V−1 s−1 by 70% under 3% strain. In-depth analysis indicates that compressing the crystal structure directly restricts the vibration of the molecules, thus suppresses dynamic disorder, a unique mechanism in organic semiconductors. Since strain can be easily induced during the fabrication process, we expect our method to be exploited to build high-performance organic devices.

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We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinoline) aluminum (Alq(3))Al interfaces by using density functional theoretical calculations, and clarified the origin of the interfacial dipole moment. We have examined various possible adsorption geometries of Alq(3) on Al surfaces and calculated the work function change induced by adsorption of Alq(3) on Al surfaces. We found that the stability depends crucially on the number of O-Al bonds formed at the interface, and Alq(3) tends to expose its O atoms to the Al substrate side and its N atoms to the vacuum side. Although the binding energies are influenced by the poor description of the van der Waals interaction by the density functionals used, the resulting bonding configurations are found to give correct binding energies when the van der Waals interaction is taken into account based on the recently proposed van der Waals density functional [Dion et al., Phys. Rev. Lett. 92, 246401 (2004)]. This bonding configuration arranges molecular permanent dipoles of Alq(3) directed towards the vacuum, leading to the decrease of the surface work function. The calculated interface dipoles agree reasonably well with the experimental results and the origin of the interface dipole formation mainly comes from the alignment of the permanent dipoles of Alq(3). The HOMO levels of the Alq(3) molecules significantly depend on the orientation of the molecular permanent dipoles and the interfacial gap state observed by experiments is ascribed to the coexistence of the two orientations of the molecular dipole moments.

Inelastically tunneled electrons from the tip of a scanning tunneling microscope were used to induce S-S bond dissociation of a (CH(3)S)(2) and lateral hopping of a CH(3)S on Cu(111) at 4.7 K. Both experimental results and theoretical calculations confirm that the excitation mechanism of the vibrationally induced chemistry reflects the projected density of states of molecular orbitals that appear near the Fermi level as a result of the rehybridization of the orbitals between the adsorbed molecules and the substrate metal atoms.