A GW+Bethe-Salpeter calculation on photoabsorption spectra of (CdSe)3 and (CdSe)6 clusters.

Photoabsorption spectra are calculated for the magic number clusters, (CdSe)(3) and (CdSe)(6), using an all-electron mixed basis GW scheme with the excitonic effect incorporated by solving the Bethe-Salpeter equation (BSE). The GW+BSE calculation provided clear size dependence of the optical gap as expected, while magnitude of the gap is overestimated compared to available experimental one. The gap is found very similarly overestimated when using the local density approximation (LDA) within the density functional theory because accidental error cancellation occurs between the significantly underestimated LDA gap and the excitonic effect neglected therein. The excitonic states are described by superposition of many one-particle states that would not be properly described within a one-particle theory, as clearly visualized in the plot of the exciton wavefunctions.

Dynamics simulation of a π-conjugated light-harvesting dendrimer II: phenylene-based dendrimer (phDG2).

We investigate the light-harvesting property of a π-conjugated dendrimer, phenylene-based dendrimer (phDG2), by carrying out a semi-classical Ehrenfest dynamics simulation based on the time-dependent density functional theory. Similar to our previous study of star-shaped stilbenoid phthalocyanine (SSS1Pc), phDG2 shows electron and hole transfer from the periphery to the core through a π-conjugated network when an electron is selectively excited in the periphery. The one-way electron and hole transfer occurs more easily in dendrimers with planar structure than in those with steric hindrance because π-conjugation is well maintained in the planar structure. The present results explain recent experiments by Akai et al (2005 J. Lumin. 112 449).

Quasiparticle energy spectra of alkali-metal clusters: all-electron first-principles calculations.

A good approximation to the one-electron self-energy operator in the calculation of quasiparticle energy spectra including the first ionization potential (IP) and electron affinity (EA) is to expand it as a simple product of a one-particle Green's function G and a dynamically screened Coulomb interaction W, namely, GW approximation. We developed a spin-polarized version of the all-electron GW approach and applied it to the first-principles calculation of quasiparticle energy spectra of alkali-metal clusters (Na(n) and K(n), n=1-8). Our all-electron mixed basis approach, in which wave functions are expressed as a linear combination of numerical atomic orbitals and plane waves, enables us to compare the absolute values of the singly (or highest) occupied molecular orbital and the lowest unoccupied molecular orbital levels with available experimental IPs and EAs. The agreements with the corresponding experimental values are fairly good. Comparing with the non-spin-polarized results of Na(2n) and K(2n) (n=1-4), we discuss the effect of spin polarization as well as the cluster size dependence of IPs or EAs.

A lattice Monte Carlo simulation of the FePt alloy using a first-principles renormalized four-body interaction.

Although a lattice Monte Carlo method provides an effective, simple, and fast way to study thermodynamic properties of substitutional alloys, it cannot treat by itself the off-lattice effects, such as thermal vibrations and local distortions. Therefore, even if the interaction among atoms at lattice points is calculated accurately by means of first-principles calculations, the lattice Monte Carlo simulation overestimates the order-disorder phase transition temperature. In this paper, we treat this problem in the investigation of the FePt alloy, which has recently attracted considerable interest in its magnetic properties. We apply a simple version of the potential renormalization theory to determine the interaction among atoms, including partly the off-lattice effects by means of first-principles calculations. Then, we use the interaction to perform a lattice Monte Carlo simulation of the FePt alloy on a fcc lattice. From the results, we find that the transition temperature obtained after the present renormalization procedure becomes closer to the experimental value.

Dynamics simulation of a π-conjugated light-harvesting dendrimer.

Carrying out a semi-classical Ehrenfest dynamics simulation based on the time-dependent density functional theory, we investigate the light-harvesting property of a π-conjugated dendrimer, star-shaped stilbenoid phthalocyanine (SSS1Pc) with oligo (p-phenylenevinylene) peripheries and show that an electron and a hole transfer from the periphery to the core through a π-conjugated network when an electron is selectively excited in the periphery. The one-way electron and hole transfer occurs more easily in dendrimers with a planar structure than in those with steric hindrance because π-conjugation is well maintained in the planar structure. The present results explain recent experiments.

Dimensional dependence of electronic structure of fullerene polymers.

Fullerene polymers made of C(60) are systematically investigated by means of a first-principles pseudopotential approach within the local density approximation of the density functional theory. We assume 10 different structures of fullerene polymers. The first three are C(60) polymer networks cross-linked by [2+2] cycloadditional four-membered rings, and the other seven are composed of peanut-shaped fused C(60) polymer chains cross-linked by either seven-membered rings or eight-membered rings. Owing to the overlap of wave functions as well as the hybrid networks of sp(2)-like (3-fold coordinated) and sp(3)-like (4-fold coordinated) carbon atoms, the electronic structure is considerably different from each other. We find that the resulting electronic structure is either semiconductor or semimetal depending on the spatial dimensionality of materials.

Two-electron distribution functions and short-range electron correlations of atoms and molecules by first principles T-matrix calculations.

The accurate first principles description of the correlations between electrons has been a topic of interest in molecular physics. We have reported in our previous paper [J. Chem. Phys. 123, 144112 (2005)] that the T matrix, which is the ladder diagrams up to the infinite order, can accurately represent the short-range electron correlations while calculating the double ionization energy spectra of atoms and molecules. In this paper, we calculate the two-electron distribution functions of real systems (Ar, CO, CO(2), and C(2)H(2)) from the eigenvalue equation associated with the Bethe-Salpeter equation for the T matrix by beginning with the local density approximation of the density functional theory and the GW approximation. We found that when the interelectron distance is very small, the Coulomb hole appears between antiparallel spin electrons due to the short-range repulsive Coulomb interaction. The resulting two-electron distribution functions clearly show the Coulomb hole.

Significant reduction of on-site Coulomb energy U due to short-range correlation in an organic Mott insulator.

By carrying out a first-principles T-matrix calculation on multiple scatterings between electrons, we show that the intramolecular electron-electron interaction energy U, of a Mott insulator of the organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is significantly reduced from the naive expectation value of the Coulomb interaction (7.3 eV and 5.3 eV, respectively, for the bare and screened Coulomb interactions) to 2.9 eV due to the short-range correlation. This result together with the intermolecular interaction energy D=1.3 eV explains the experimental optical gap (1.5 eV). The associated two-particle wavefunction clearly shows the Coulomb hole indicating that two electrons with antiparallel spins cannot approach because of the Coulomb repulsion. We also discuss the energetics and magnetics of this system.

First-principles T-matrix calculations of double-ionization energy spectra of atoms and molecules.

Strong electron correlation plays an important role in the determination of double ionization energy, which is required for removing or adding two electrons, particularly in small-sized systems. Starting from the state-of-the-art GW approximation, we evaluate the particle-particle ladder diagrams up to the infinite order by solving the Bethe-Salpeter equation of the T-matrix theory to calculate the double-ionization energy spectra of atoms and molecules (Be, Mg, Ca, Ne, Ar, Kr, CO, C(2)H(2), Li(2), Na(2), and K(2)) from first principles. The ladder diagrams up to the infinite order are significant to calculations of double-ionization energy spectra. The present results are in good agreement with available experimental data as well as the previous calculations using, e.g., the configuration-interaction method.

Evolution of the electronic structure of Be clusters.

Using a modified symbiotic genetic algorithm approach and many-body interatomic potential derived from first principles, we have calculated equilibrium geometries and binding energies of the ground-state and low-lying isomers of Be clusters containing up to 41 atoms. Molecular-dynamics study was also carried out to study the frequency of occurrence of the various geometrical isomers as these clusters are annealed during the simulation process. For a selected group of these clusters, higher-energy isomers were more often found than their ground-state structures due to large catchment areas. The accuracy of the above ground-state geometries and their corresponding binding energies were verified by carrying out separate ab initio calculations based on molecular-orbital approach and density-functional theory with generalized gradient approximation for exchange and correlation. The atomic orbitals were represented by a Gaussian 6-311G** basis, and the geometry optimization was carried out using the GAUSSIAN 98 code without any symmetry constraint. While the ground-state geometries and their corresponding binding energies obtained from ab initio calculations do not differ much from those obtained using the molecular-dynamics approach, the relative stability of the clusters and the energy gap between the highest occupied and the lowest unoccupied molecular orbitals show significant differences. The energy gaps, calculated using the density-functional theory, show distinct shell closure effects, namely, sharp drops in their values for Be clusters containing 2, 8, 20, 34, and 40 electrons. While these features may suggest that small Be clusters behave free-electron-like and, hence, are metallic, the evolution of the structure, binding energies, coordination numbers, and nearest-neighbor distances do not show any sign of convergence towards the bulk value. We also conclude that molecular-dynamics simulation based on many-body interatomic potentials may not always give the correct picture of the evolution of the structure and energetics of clusters although they may serve as a useful tool for obtaining starting geometries by efficiently searching a large part of the phase space.