Giovanni Onida

A brief introduction to the ABINIT software package

Abstract A brief introduction to the ABINIT software package is given. Available under a free software license, it allows to compute directly a large set of properties useful for solid state studies, including structural and elastic properties, prediction of phase (meta)stability or instability, specific heat and free energy, spectroscopic and vibrational properties. These are described, and corresponding applications are presented. The emphasis is also laid on its ease of use and extensive documentation, allowing newcomers to quickly step in.

Ab Initio Calculation of Excitonic Effects in the Optical Spectra of Semiconductors

An ab initio approach to the calculation of excitonic effects in the optical absorption spectra of semiconductors and insulators is formulated. It starts from a quasiparticle band structure calculation and is based on the relevant Bethe-Salpeter equation. An application to bulk silicon shows a substantial improvement with respect to previous calculations in the description of the experimental spectrum, for both peak positions and line shape.

Excitonic effects in solids described by time-dependent density-functional theory

Starting from the many-body Bethe-Salpeter equation we derive an exchange-correlation kernel f(xc) that reproduces excitonic effects in bulk materials within time-dependent density functional theory. The resulting f(xc) accounts for both self-energy corrections and the electron-hole interaction. It is static, nonlocal, and has a long-range Coulomb tail. Taking the example of bulk silicon, we show that the -alpha/q(2) divergency is crucial and can, in the case of continuum excitons, even be sufficient for reproducing the excitonic effects and yielding excellent agreement between the calculated and the experimental absorption spectrum.

Quantum Spin Transport in Carbon Chains

First-principles and non-equilibrium Greens function approaches are used to predict spin-polarized electronic transport in monatomic carbon chains covalently connected to graphene nanoribbons, as recently synthetized experimentally (Jin, C.; et al. Phys. Rev. Lett. 2009, 102, 205501-205504). Quantum electron conductances exhibit narrow resonant states resulting from the simultaneous presence of open conductance channels in the contact region and on the chain atoms. Odd-numbered chains, which acquire metallic or semiconducting character depending on the nature of the edge at the graphene contact, always display a net spin polarization. The combination of electrical and magnetic properties of chains and contacts results in nanodevices with intriguing spintronic properties such as the coexistence of magnetic and semiconducting behaviors.

Quasiparticle electronic structure of copper in the GW approximation

We show that the results of photoemission and inverse photoemission experiments on bulk copper can be quantitatively described within band-structure theory, with no evidence of effects beyond the single-quasiparticle approximation. The well-known discrepancies between the experimental band structure and the Kohn-Sham eigenvalues of density functional theory are almost completely corrected by self-energy effects. Exchange-correlation contributions to the self-energy arising from 3s and 3p core levels are shown to be crucial.

Double excitations in finite systems.

Time-dependent density-functional theory (TDDFT) is widely used in the study of linear response properties of finite systems. However, there are difficulties in properly describing excited states, which have double- and higher-excitation characters, which are particularly important in molecules with an open-shell ground state. These states would be described if the exact TDDFT kernel were used; however, within the adiabatic approximation to the exchange-correlation (xc) kernel, the calculated excitation energies have a strict single-excitation character and are fewer than the real ones. A frequency-dependent xc kernel could create extra poles in the response function, which would describe states with a multiple-excitation character. We introduce a frequency-dependent xc kernel, which can reproduce, within TDDFT, double excitations in finite systems. In order to achieve this, we use the Bethe-Salpeter equation with a dynamically screened Coulomb interaction W(omega), which can describe these excitations, and from this we obtain the xc kernel. Using a two-electron model system, we show that the frequency dependence of W does indeed introduce the double excitations that are instead absent in any static approximation of the electron-hole screening.

Optical properties of BN in cubic and layered hexagonal phases

INFM Sezione di Roma-2 and Dipartimento di Fisica, Universita`di Tor Vergata, Via della Ricerca Scientifica 1, I-00133, Rome, Italy~Received 24 January 2001; revised manuscript received 6 April 2001; published 26 June 2001!Linear optical functions of cubic and hexagonal BN have been studied within first principles density func-tional theory in the local density approximation. Calculated energy-loss functions show reasonable agreementwith experiments and previous theoretical results both for h-BN and forc-BN. Discrepancies arise betweentheoretical results and experiments in the imaginary part of the dielectric function for c-BN. Possible expla-nations of this mismatch are proposed and evaluated: lattice constant variations, h-BN contamination inc-BNsamples, and self-energy effects.DOI: 10.1103/PhysRevB.64.035104 PACS number~s!: 78.20.Ci, 68.35.Ja, 71.45.GmI. INTRODUCTION

Vibrational Spectrum of C60: A Bond-Charge Model Calculation

We present a calculation of the vibrational spectrum of the C60 cluster by means of an adiabatic bond-charge model, which is directly derived from a calculation reproducing the intralayer phonons in crystalline graphite. Despite the very restricted number of adjustable parameters (four), the fourteen experimental Raman and infrared-active frequencies are reproduced within 3.0% (mean-square relative deviation). Good agreement is found also for other optically inactive modes, recently detected by neutron scattering and reflection electron energy loss spectroscopy.

AB INITIO OPTICAL PROPERTIES OF SI(100)

We compute the linear optical properties of different reconstructions of the clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving pseudopotentials. The equilibrium atomic geometries of the surfaces, determined from self-consistent total-energy calculations within the Car-Parrinello scheme, strongly influence reflectance anisotropy spectra, showing differences between the

First-principles calculation of the plasmon resonance and of the reflectance spectrum of silver in the GW approximation

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