Susumu Yanagisawa

A long-range-corrected time-dependent density functional theory

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.

An investigation of density functionals: The first-row transition metal dimer calculations

The performance of different density functional theory (DFT) 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.

State-selective dissociation of a single water molecule on an ultrathin MgO film

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.

Density functional theoretical study of pentacene/noble metal interfaces with van der Waals corrections: Vacuum level shifts and electronic structures

In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu(111), Ag(111), and Au(111) by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals (vdW) approach [S. Grimme, J. Comput. Chem. 27, 1787 (2006)] 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) > or = 0.35 nm, the work function changes (Delta phis) do not depend on the substrate work function (phi(m)), indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z(C) < or = 0.25 nm, Delta phis vary nearly linearly with phi(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.

First-principles theoretical study of Alq3∕Al interfaces: Origin of the interfacial dipole

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.

Investigation of the use of density functionals in second- and third-row transition metal dimer calculations

We explore the use of density functionals in calculating the equilibrium distances, dissociation energies, and harmonic vibrational frequencies of the homonuclear diatomics of the second‐row transition metals, platinum, and gold. The outermost s–d interconfigurational energies (ICEs) and the outermost s and d ionization potentials (IPs) were also calculated for the second‐ and third‐row transition metal atoms. Compared with the first‐row transition metal dimer calculations (J Chem Phys 2000, 112, 545–553), the binding energies calculated using the combination of the Becke 1988 exchange and the one‐parameter progressive correlation (BOP) functional and Beckes three‐parameter hybrid (B3LYP) functional are in better agreement with the experiment. However, the pure BOP functional still gives the deep and narrow dissociation potential wells for the electron configurations containing high‐angular‐momentum open‐shell orbitals. Analysis of the s–d ICEs and the s and d IPs suggests that the overestimation may be due to the insufficient long‐range interaction between the outermost s and d orbitals in the exchange functional. The hybrid B3LYP functional seems to partly solve this problem for many systems by the incorporation of the Hartree–Fock exchange integral. However, this still leads to an erroneous energy gap between the configurations of fairly different spin multiplicity, probably because of the unbalance of exchange and correlation contributions.

Theoretical Investigation on the Electronic Structure of the Tris-(8-hydroxyquinolinato) Aluminum/Aluminum Interface

We have studied the atomic geometries and the electronic properties of the tris-(8-hydroxyquinolinato) aluminum (Alq3)/Al interface by using density functional theoretical calculations. We examined three surfaces, the close packed Al(111) surface, the Al(332) stepped surface, and the Al adatom adsorbed Al(111) surface to investigate the effect of the surface roughness on the electronic properties of the interfaces. The calculated interface dipoles agree reasonably well with the experimental results and we found that the origin of the interface dipole formation mainly comes from the permanent dipole moment of Alq3 molecules. Although we have examined various possible structures, an interface gap state observed experimentally could not be reproduced by the present calculations.

Scanning tunneling microscopy/spectroscopy of picene thin films formed on Ag(111)

Using ultrahigh-vacuum low-temperature scanning tunneling microscopy and spectroscopy combined with first principles density functional theory calculations, we have investigated structural and electronic properties of pristine and potassium (K)-deposited picene thin films formed in situ on a Ag(111) substrate. At low coverages, the molecules are uniformly distributed with the long axis aligned along the [112̄] direction of the substrate. At higher coverages, ordered structures composed of monolayer molecules are observed, one of which is a monolayer with tilted and flat-lying molecules resembling a (11̄0) plane of the bulk crystalline picene. Between the molecules and the substrate, the van der Waals interaction is dominant with negligible hybridization between their electronic states; a conclusion that contrasts with the chemisorption exhibited by pentacene molecules on the same substrate. We also observed a monolayer picene thin film in which all molecules were standing to form an intermolecular π stacking. Two-dimensional delocalized electronic states are found on the K-deposited π stacking structure.

The relativistic effect on energies of light elements: a RESC-BOP study

Abstract The scalar relativistic contribution to energies and geometries of molecules containing the first- and second-row atoms is estimated by the density functional theory with the BOP functional. The relativistic effect is considered through the relativistic scheme by eliminating small-component (RESC) of the four-component Dirac equation. The relativistic corrections to the energies of light elements are rather small but show a clear-cut trend. The scalar relativistic effect nearly always reduces atomization energies, ionization potentials, and electron affinities. Furthermore, the relativistic corrections to the ionization potentials and electron affinities increase with the atomic numbers in the component atoms. On the other hand, there is little change in the geometric parameters of the first- and second-row diatomic molecules.

Tensile-strain effect of inducing the indirect-to-direct band-gap transition and reducing the band-gap energy of Ge

By means of a hybrid density-functional method, we investigate the tensile-strain effect of inducing the indirect-to-direct band-gap transition and reducing the band-gap energy of Ge. We consider [001], [111], and [110] uniaxial tensility and (001), (111), and (110) biaxial tensility. Under the condition of no normal stress, we determine both normal compression and internal strain, namely, relative displacement of two atoms in the primitive unit cell, by minimizing the total energy. We identify those strain types which can induce the band-gap transition, and evaluate the critical strain coefficient where the gap transition occurs. Either normal compression or internal strain operates unfavorably to induce the gap transition, which raises the critical strain coefficient or even blocks the transition. We also examine how each type of tensile strain decreases the band-gap energy, depending on its orientation. Our analysis clearly shows that synergistic operation of strain orientation and band anisotropy has a great influence on the gap transition and the gap energy.