Session P31 - Density Functional Theory: Time-dependent and Clusters.
ORAL session, Wednesday morning, March 05
Room 19A, Austin Convention Center

[P31.001] Time-Dependent Density Functional Theory - Efficient Algorithms for Atoms, Molecules and Solids

Time-dependent density functional theory represents a powerful method for treating the dynamic response of electrons and ions in the presence of time-dependent external fields, e.g., laser light. Most implementations so far have been in the linear regime, probing the "excited states" of molecules, nanoparticles, and solids. There are only a few implementations in the full nonlinear regime where the effect of intense laser light can be treated quantitatively.[1] Here we report the results of an investigation of several techniques for time-stepping algorithms for simultaneous electron and ion dynamics, which have been implemented, tested, and compared for stability, efficiency, and accuracy. Earlier results are reproduced and new applications will be reported on high-harmonic generation in atoms, molecules and solids, second-harmonic generation at solid interfaces, and other phenomena induced by intense ultrashort laser pulses.

[1] J.J. Vincente Alvarez, M. Ferconi, and S.T. Pantelides, \itLaser \itand \itParticle \itBeams. \bf18, 557-562 (2000).

[P31.002] Excitation energies from TDDFT beyond the adiabatic approximation

Time-dependent density functional theory (TDDFT) in the linear-response regime is a very successful method for calculating electronic excitation energies in atoms and molecules. The standard TDDFT approach uses the adiabatic approximation for exchange and correlation (xc), i.e., neglects the frequency dependence of the linearized xc potential. We present a time-dependent current-DFT (TCDFT) formalism for excitation energies that includes non-adiabatic and retardation effects through a frequency-dependent xc vector potential. The formalism is used to obtain non-adiabatic corrections to excitation energies of closed-shell atoms. The results illustrate why the adiabatic approximation works so well for atoms and molecules.

[P31.003] Solving the ultra-nonlocality problem in time-dependent spin-density functional theory

It has been known for some time that the exchange-correlation potential in time-dependent density functional theory is an ``ultranonlocal" functional of the density.  A much more severe nonlocality problem, with a completely different physical origin, plagues the exchange-correlation potentials in time-dependent spin-density functional theory.  We show how the use of the spin current density as a basic variable solves this problem, and provide explicit expressions for the exchange-correlation fields as functionals of the spin currents.

[P31.004] Excitonic Optical Spectrum of Semiconductors Obtained by Time-Dependent Density-Functional Theory with the Exact-Exchange Kernel

Applying a novel exact-exchange (EXX) approach within time-dependent density-functional theory, we obtained the optical absorption spectrum of bulk silicon in good agreement with experiments including excitonic features. Analysis of the EXX kernel shows that inclusion of the Coulomb coupling of electron-hole pairs and the correct long-wavelength behavior in the kernel is crucial for the proper description of excitonic effects in semiconductors.

[P31.005] Absorption Spectra and Optical Gaps of Small Hydrogenated Silicon Dots: Comparison of Theory and Experiment

The optical properties of semiconductor quantum dots have long been a subject of intensive experimental and theoretical research. However, a comparison of different theoretical and experimental results has been complicated by diverse and often inconsistent definitions of the optical absorption gaps and optically allowed / optically forbidden transitions used in different works. In this talk, I psesent time-dependent density functional calculations for the absorption spectra of small hydrogenated silicon clusters. The structures of the theoretical spectra are analyzed in detail and the calculated absorption peaks are matched to the available experimental data based on the symmetry of individual electronic transitions. I also review and compare common experimental and theoretical definitions of the optical gaps, as well as dipole-allowed and dipole-forbidden transitions and discuss their applicability to small semiconductor dots.

[P31.006] Generalized LDA+U Functional

We propose a new form of the LDA+U functional consistent with the local density functional theory. Our approach is based on the idea that DFT by construction reproduces the exact ground state energy, so the LDA+U correction to the total energy should vanish when the double counted terms are properly handled. The proposed functional can be formulated as a constrained LDA theory. It reduces to two most commonly used LDA+U functionals in the limiting cases of uniform occupancies and strongly localized orbitals. We apply our method to strongly (e.g. 4f compounds, NiO) and weakly (e.g. FeAl) correlated systems and compare it with existing recipes for LDA+U calculations from the point of view of (i) their first-principles justification, (ii) their practical usefulness, and (iii) their effect on magnetic properties. We also discuss a relatively new area of applying LDA+U to moderately-correlated, metallic systems. Our analysis shows that none of the LDA+U functionals correctly describes the essential physics of the weakly correlated metals such as FeAl: (i) reducing the band dispersion by dressing one-particle excitation, and (ii) spin fluctuations near the quantum critical point. By comparing with dynamical mean field calculations, we show that the suppression of magnetism in FeAl is due to the spin fluctuations near the quantum critical point and cannot be explained within the LDA+U. Finally, we discuss usage of LDA+U functional in the systems with strong spin-orbit interaction.

[P31.007] Dynamical Charge Response of NiO

We report an all-electron evaluation of the spectrum of charge density excitations of NiO within the framework of time-dependent density functional theory. We start out from a self-consistent LDA+U ground state which corresponds to a charge-transfer insulator. The charge responses are obtained with use of an adiabatic approximation for the corresponding many-body kernels. The calculated momentum-resolved spectra of charge response agree well with inelastic X-ray scattering data, which we took at the Advanced Photon Source. In the optical limit, the observed low-energy loss structures are dominated by charge fluctuations involving the occupied O-p and the empty Ni-d states via their hybridization tails.

[P31.008] Giant Magnetic Moments of Nitrogen Doped Mn Clusters and Their Relevance to Ferromagnetism in Mn Doped GaN

Calculations based on density functional theory show that the stability and magnetic properties of small Mn clusters can be fundamentally altered by the presence of nitrogen. Not only are their binding energies substantially enhanced, but also the coupling between the magnetic moments at Mn sites remain ferromagnetic irrespective of their size or shape. In addition, these nitrogen doped Mn clusters carry giant magnetic moments ranging from 4 \mu_B in MnN to 22 \mu_B in Mn_5N. In particular, the 5.0 \mu_B magnetic moment of Mn_7 is enhanced by more than a factor of 4 by attaching a nitrogen atom. It is suggested that the giant magnetic moments of MnxN clusters may play a key role in the ferromagnetism of Mn doped GaN which exhibit a wide range (10K - 940K) of Curie temperatures.

[P31.009] Atomic geometry, electronic structure, and magnetism of 13-atom metal clusters

Atomic geometries, electronic structures, and magnetic moments of several metal clusters with 13 atoms are studied by ab initio density-functional calculations. The ground state structures of 13-atom metal clusters were previously assumed to be icosahedron, cuboctahedron, or decahedron. However, in this study, using ab initio molecular dynamics simulations, we find another low-lying energy state with a buckled bi-planar structure that has C_2v symmetry. The spin magnetic moments for the buckled bi-planar structure are usually lower than for the icosahedral structure, which are more consistent with existing experimentally measured values. This novel buckled bi-planar structure of 13-atom cluster should be a common low-lying energy state for the late transition metals (with d electrons more than half filled).

[P31.010] Evolution of the Electronic Structure of Be Clusters

Using a modified symbiotic genetic algorithm approach and many body inter-atomic 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. The accuracy of the geometries and the binding energies were verified by carrying out separate ab-initio calculations based on self-consistent field-linear combinations of atomic orbitals molecular orbital (SCF-LCAO-MO) method and density functional theory with generalized gradient approximation for exchange and correlation. 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 and the genetic algorithm, the relative stability of the clusters and the energy gap between the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals show significant differences. The SCF-LCAO-MO results show distinct shell closure effects at 2, 8, 20, 34, and 40 electrons.

[P31.011] All-electron GW quasiparticle energies of small silicon clusters

We perform ab-initio quasiparticle(QP) energy calculations for small silicon clusters(Si_n,n=4,5,6) within the GW approximation. In the calculations we employ an all-electron mixed-basis approach using plane waves and atomic orbitals as a basis set. Obtained GW QP energies are compared with experimental ionization potentials (IPs) and electron affinities(EAs). IPs are in good agreement with experimental values. For EAs, which is very sensitive to the ionic valence and cluster geometry, it is very important to consider photoemission process. All results are in good agreements with the experimental data.

[P31.012] Geometry and Electronic Structure of V_n(Benzene)_m complexes

The equilibrium geometries and binding energies of neutral, positively and negatively charged vanadium-benzene complexes(V_nBz_n+1, n=1-3) have been calculated using density functional theory and generalized gradient approximation for exchange and correlation functional. The global optimization was first carried out by using the LanL2dz basis and the Gaussian 98 code for all possible spin configurations with out any symmetry constraint. The geometries corresponding to the ground state spin multiplicity were further re-optimized using all electron 6-311G* basis but keeping the preferred spin and symmetry in the earlier calculation. The calculated electron affinities and magnetic moments will be compared with the most recent experimental data using photo-electron spectroscopy and deflections in the Stern-Gerlach magnetic field.

[P31.013] Surface Plasmon Enhanced Optical Forces in Silver Nano-aggregates

We use extended Mie theory to investigate optical forces induced by and acting on small silver nanoparticle aggregates excited at surface plasmon resonance. It is shown that single molecules can be trapped at junctions between closely spaced nanoparticles, which are simultaneously pulled together by optical forces. These effects could significantly influence surface-enhanced Raman scattering and related spectroscopies under normal experimental conditions, and contribute to single-molecule sensitivity.

[P31.014] ELECTRONIC STRUCTURE, STABILITY AND VIBRATIONAL PROPERTIES OF TI_8C_12 AND ITS DIMER

The ground state geometry of Ti_8C_12 is investigated within density functional formalism. Our calculations show that the lowest energy structure has C_3v symmetry. We also compare our calculated infrared absorption intensities for the pure Ti_8C_12 cluster with the available experimentally measured spectrum. The thermal effects on the measured spectrum are explored in terms of the spectrum of the high energy conformers. We also show the neutral Ti_8C_12 can form stable dimers with a binding energy of 4.8 eV. The calculation of vibrational frequencies indicate the dimer structure as a local minima on the potential energy surface. The infrared absorption and Raman scattering spectra of the dimer will be presented. The possibilities for detection of the dimer cation in the time-of-flight mass spectroscopy or the neutral dimer in a ligand-rich environment will also be discussed.

[P31.015] Strusture and Properties of Nitrogen Doped Small Cr Clusters

Cr has an anti-ferromagnetic bulk phase. However, small clusters of Cr show a rich variety of magnetic behavior. The coupling between magnetic moments ranges from antiferromagnetic to ferrimagnetic and even ferromagnetic. Isomers of a cluster may often exhibit varying magnetic moments. We show that doping of nitrogen has a dramatic influence on the magnetic coupling in pure Cr clusters. For example, the magnetic moment of Cr_3N is almost an order of magnitude larger than that of Cr_3 while the doping has negligible effect in Cr_5N. These results based on density functional calculations indicate that if clustering of Cr around a nitrogen atom in GaN can be suitably controlled, it may be possible to achieve a ferromagnetic semi-conductor with a high Curie temperature.

Part P of program listing