Molecular electronic excitations calculated from a solid-state approach: Methodology and numerics

Hahn, P. H.; Schmidt, Wolf Gero; Bechstedt, F.

doi:10.1103/physrevb.72.245425

Cited by 67 publications

(59 citation statements)

References 103 publications

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“…At the same time, significant effort has been dedicated on the theory side. Some of the difficulties one may encounter by applying the GW approximation to large molecules were emphasized recently 15,16 . So far, reliable calculations of electronic self-energy within the GW approximation have been done for confined systems with up to a few tens of atoms 8,9,17,18,19 .…”

Section: Introductionmentioning

confidence: 99%

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Tiago¹,

2006

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Spectroscopic and optical properties of nanosystems and point defects are discussed within the framework of Green's function methods. We use an approach based on evaluating the self-energy in the so-called GW approximation and solving the Bethe-Salpeter equation in the space of singleparticle transitions. Plasmon-pole models or numerical energy integration, which have been used in most of the previous GW calculations, are not used. Fourier transforms of the dielectric function are also avoided. This approach is applied to benzene, naphthalene, passivated silicon clusters (containing more than one hundred atoms), and the F center in LiCl. In the latter, excitonic effects and the 1s → 2p defect line are identified in the energy-resolved dielectric function. We also compare optical spectra obtained by solving the Bethe-Salpeter equation and by using time-dependent density functional theory in the local, adiabatic approximation. From this comparison, we conclude that both methods give similar predictions for optical excitations in benzene and naphthalene, but they differ in the spectra of small silicon clusters. As cluster size increases, both methods predict very low cross section for photoabsorption in the optical and near ultra-violet ranges. For the larger clusters, the computed cross section shows a slow increase as function of photon frequency. Ionization potentials and electron affinities of molecules and clusters are also calculated.

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Section: Introductionmentioning

confidence: 99%

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Tiago¹,

2006

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“…Path integral quantum Monte Carlo methods are also under development. These methods should prove valuable when considering matter in extreme conditions [397,398]. Many of these computational methods will benefit from the ongoing rapid improvements in computing capabilities.…”

Section: Opportunities For Theorymentioning

confidence: 99%

Vibrational thermodynamics of materials

Fultz¹

2010

Progress in Materials Science

570

428

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Abstract. The literature on vibrational thermodynamics of materials is reviewed. The emphasis is on metals and alloys, especially on the progress over the last decade in understanding differences in the vibrational entropy of different alloy phases and phase transformations. Some results on carbides, nitrides, oxides, hydrides and lithium-storage materials are also covered.Principles of harmonic phonons in alloys are organized into thermodynamic models for unmixing and ordering transformations on an Ising lattice, and extended for non-harmonic potentials. Owing to the high accuracy required for the phonon frequencies, quantitative predictions of vibrational entropy with analytical models prove elusive. Accurate tools for such calculations or measurements were challenging for many years, but are more accessible today. Ab-initio methods for calculating phonons in solids are summarized. The experimental techniques of calorimetry, inelastic neutron scattering, and inelastic x-ray scattering are explained with enough detail to show the issues of using these methods for investigations of vibrational thermodynamics. The explanations extend to methods of data analysis that affect the accuracy of thermodynamic information.It is sometimes possible to identify the structural and chemical origins of the differences in vibrational entropy of materials, and the number of these assessments is growing. There has been considerable progress in our understanding of the vibrational entropy of mixing in solid solutions, compound formation from pure elements, chemical unmixing of alloys, order-disorder transformations, and martensitic transformations. Systematic trends are available for some of these phase transformations, although more examples are needed, and many results are less reliable at high temperatures. Nanostructures in materials can alter sufficiently the vibrational dynamics to affect thermodynamic stability. Internal stresses in polycrystals of anisotropic materials also contribute to the heat capacity. Lanthanides and actinides show a complex interplay of vibrational, electronic, and magnetic entropy, even at low temperatures.A "quasiharmonic model" is often used to extend the systematics of harmonic phonons to high temperatures by accounting for the effects of thermal expansion against a bulk modulus. Non-harmonic effects beyond the quasiharmonic approximation originate from the interactions of thermally-excited phonons with other phonons, or with the interactions of phonons with electronic excitations. In the classical high temperature limit, the adiabatic electron-phonon coupling can have a surprisingly large effect in metals when temperature causes significant changes in the electron density near the Fermi level. There are useful similarities in how temperature, pressure, and composition alter the conduction electron screening and the interatomic force constants. Phononphonon "anharmonic" interactions arise from those non-harmonic parts of the interatomic potential that cannot be accounted for by the quasiharmonic ...

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“…This especially holds for their reduction due to the confinement in the NC cores which has been experimentally confirmed 37 . In order to describe the correct absolute positions of the energy levels quasiparticle (QP) corrections of the KohnSham eigenvalues are recently also taken into account for Si nanocrystals 38,39 . Thereby, the exchange-correlation (XC) self-energy is approximated in the framework of Hedin's GW approximation 40 .…”

Section: B Electronic Structuresmentioning

confidence: 99%

Tunneling of electrons between Si nanocrystals embedded in a SiO2matrix

2012

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Based on ab initio density functional calculations of Si nanocrystals (NCs) in a SiO2 matrix we study the impact of NC size and separation on electronic properties and present consequences for inter-NC carrier transport. The nanocrystal size and separation significantly influence the electronic structure of three-dimensional superlattice systems with different NC filling. The energy levels for large NC distances are broadened to minibands for small separations due to the wave-function overlap. Their formation is described in terms of inter-NC hopping and carrier tunneling. We find that electron transport in conduction minibands is more likely than hole transport.

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Molecular electronic excitations calculated from a solid-state approach: Methodology and numerics

Cited by 67 publications

References 103 publications

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Optical excitations in organic molecules, clusters, and defects studied by first-principles Green’s function methods

Vibrational thermodynamics of materials

Tunneling of electrons between Si nanocrystals embedded in a SiO2matrix

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