Special Issue "50th Anniversary of the Kohn-Sham Theory—Advances in Density Functional Theory"

A special issue of Computation (ISSN 2079-3197). This special issue belongs to the section "Computational Chemistry".

Deadline for manuscript submissions: closed (31 May 2016)

Special Issue Editors

Guest Editor
Prof. Dr. Karlheinz Schwarz

Theoretical Chemistry Group, Material Chemisty, TU Wien, Technical University Vienna, A-1060 Vienna, Austria
Website | E-Mail
Phone: +43 1 58801 165301
Fax: +43 1 58801 165982
Interests: density functional theory (DFT); electronic structure of solids and surfaces; chemical bonding; spectra; high performance computing; Wien2k code
Guest Editor
Prof. Dr. Agnes Nagy

Department of Physics, University of Debrecen, H-4010 Debrecen, Hungary
Website | E-Mail
Interests: density functional theory; pair density functional theory; information concepts

Special Issue Information

Dear Colleagues,

This Special Issue will consist of selected excellent papers from the 16th International Conference on “Density Functional Theory and Its Applications” (DFT2015), which will be held in Debrecen, Hungary, from the 31 August to 4 September 2015. This international conference will celebrate the 50th anniversary of the Kohn-Sham theory and is designed to share a wide variety of ideas. Contributors will be invited to submit and present papers concerning the “concepts and applications”: Topics of selected papers will include various method developments and applications for molecules and solids.

This Special Issue is dedicated to demonstrating recent advances in studying the electronic structure for a variety of systems in their ground state or excited state, from regular to highly correlated systems. Papers may report on original research, discuss methodological aspects, review the current state-of-the-art, or offer perspectives on future prospects.

These papers will be subjected to peer review and are published so as to widely disseminate new research results, including developments and applications.

The authors of papers submitted to the 16th DFT2015 conference (http://dft2015.unideb.hu) will be given the opportunity to submit extended versions of their works in this Special Issue, provided they fulfill the specific journal requirements found at https://www.mdpi.com/journal/computation/instructions.

Specific methods and fields of applications include, but are not limited to:

  • Time independent excited state density functional theory
  • Time dependent density functional theory
  • Density functional theory of solids
  • Strongly correlated systems
  • Biomolecular modeling and bioreactions

Prof. Dr. Karlheinz Schwarz
Prof. Dr. Agnes Nagy
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Computation is an international peer-reviewed open access quarterly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 350 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.



Keywords

  • Density functional theory (DFT)
  • Time dependent density functional theory (TDDFT)
  • Electronic structure
  • DFT functional
  • Structure property relations
  • (Bio)molecules and solids

Published Papers (17 papers)

View options order results:
result details:
Displaying articles 1-17
Export citation of selected articles as:

Editorial

Jump to: Research

Open AccessEditorial Special Issue “50th Anniversary of the Kohn–Sham Theory—Advances in Density Functional Theory”
Computation 2016, 4(4), 45; https://doi.org/10.3390/computation4040045
Received: 15 November 2016 / Revised: 17 November 2016 / Accepted: 17 November 2016 / Published: 22 November 2016
PDF Full-text (684 KB) | HTML Full-text | XML Full-text
Abstract
The properties of many materials at the atomic scale depend on the electronic structure, which requires a quantum mechanical treatment. The most widely used approach to make such a treatment feasible is density functional theory (DFT), the advances in which were presented and
[...] Read more.
The properties of many materials at the atomic scale depend on the electronic structure, which requires a quantum mechanical treatment. The most widely used approach to make such a treatment feasible is density functional theory (DFT), the advances in which were presented and discussed during the DFT conference in Debrecen. Some of these issues are presented in this Special Issue. Full article
Figures

Figure 1

Open AccessEditorial Obituary for Walter Kohn (1923–2016)
Computation 2016, 4(4), 40; https://doi.org/10.3390/computation4040040
Received: 8 October 2016 / Revised: 13 October 2016 / Accepted: 14 October 2016 / Published: 20 October 2016
Cited by 2 | PDF Full-text (1043 KB) | HTML Full-text | XML Full-text
Abstract
Walter Kohn (Figure 1) is one of the most cited scientists of our time, who died on 19 April 2016 in Santa Barbara, CA, USA. [...] Full article
Figures

Figure 1

Research

Jump to: Editorial

Open AccessArticle Towards TDDFT for Strongly Correlated Materials
Computation 2016, 4(3), 34; https://doi.org/10.3390/computation4030034
Received: 31 May 2016 / Revised: 23 August 2016 / Accepted: 1 September 2016 / Published: 10 September 2016
Cited by 1 | PDF Full-text (1657 KB) | HTML Full-text | XML Full-text
Abstract
We present some details of our recently-proposed Time-Dependent Density-Functional Theory (TDDFT) for strongly-correlated materials in which the exchange-correlation (XC) kernel is derived from the charge susceptibility obtained using Dynamical Mean-Field Theory (the TDDFT + DMFT approach). We proceed with deriving the expression for
[...] Read more.
We present some details of our recently-proposed Time-Dependent Density-Functional Theory (TDDFT) for strongly-correlated materials in which the exchange-correlation (XC) kernel is derived from the charge susceptibility obtained using Dynamical Mean-Field Theory (the TDDFT + DMFT approach). We proceed with deriving the expression for the XC kernel for the one-band Hubbard model by solving DMFT equations via two approaches, the Hirsch–Fye Quantum Monte Carlo (HF-QMC) and an approximate low-cost perturbation theory approach, and demonstrate that the latter gives results that are comparable to the exact HF-QMC solution. Furthermore, through a variety of applications, we propose a simple analytical formula for the XC kernel. Additionally, we use the exact and approximate kernels to examine the nonhomogeneous ultrafast response of two systems: a one-band Hubbard model and a Mott insulator YTiO3. We show that the frequency dependence of the kernel, i.e., memory effects, is important for dynamics at the femtosecond timescale. We also conclude that strong correlations lead to the presence of beats in the time-dependent electric conductivity in YTiO3, a feature that could be tested experimentally and that could help validate the few approximations used in our formulation. We conclude by proposing an algorithm for the generalization of the theory to non-linear response. Full article
Figures

Figure 1

Open AccessArticle The Influence of One-Electron Self-Interaction on d-Electrons
Computation 2016, 4(3), 33; https://doi.org/10.3390/computation4030033
Received: 31 May 2016 / Revised: 25 August 2016 / Accepted: 31 August 2016 / Published: 6 September 2016
Cited by 6 | PDF Full-text (759 KB) | HTML Full-text | XML Full-text
Abstract
We investigate four diatomic molecules containing transition metals using two variants of hybrid functionals. We compare global hybrid functionals that only partially counteract self-interaction to local hybrid functionals that are designed to be formally free from one-electron self-interaction. As d-orbitals are prone
[...] Read more.
We investigate four diatomic molecules containing transition metals using two variants of hybrid functionals. We compare global hybrid functionals that only partially counteract self-interaction to local hybrid functionals that are designed to be formally free from one-electron self-interaction. As d-orbitals are prone to be particularly strongly influenced by self-interaction errors, one may have expected that self-interaction-free local hybrid functionals lead to a qualitatively different Kohn–Sham density of states than global hybrid functionals. Yet, we find that both types of hybrids lead to a very similar density of states. For both global and local hybrids alike, the intrinsic amount of exact exchange plays the dominant role in counteracting electronic self-interaction, whereas being formally free from one-electron self-interaction seems to be of lesser importance. Full article
Figures

Figure 1

Open AccessArticle Electron Correlations in Local Effective Potential Theory
Computation 2016, 4(3), 30; https://doi.org/10.3390/computation4030030
Received: 22 June 2016 / Revised: 8 August 2016 / Accepted: 10 August 2016 / Published: 16 August 2016
Cited by 7 | PDF Full-text (348 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Local effective potential theory, both stationary-state and time-dependent, constitutes the mapping from a system of electrons in an external field to one of the noninteracting fermions possessing the same basic variable such as the density, thereby enabling the determination of the energy and
[...] Read more.
Local effective potential theory, both stationary-state and time-dependent, constitutes the mapping from a system of electrons in an external field to one of the noninteracting fermions possessing the same basic variable such as the density, thereby enabling the determination of the energy and other properties of the electronic system. This paper is a description via Quantal Density Functional Theory (QDFT) of the electron correlations that must be accounted for in such a mapping. It is proved through QDFT that independent of the form of external field, (a) it is possible to map to a model system possessing all the basic variables; and that (b) with the requirement that the model fermions are subject to the same external fields, the only correlations that must be considered are those due to the Pauli exclusion principle, Coulomb repulsion, and Correlation–Kinetic effects. The cases of both a static and time-dependent electromagnetic field, for which the basic variables are the density and physical current density, are considered. The examples of solely an external electrostatic or time-dependent electric field constitute special cases. An efficacious unification in terms of electron correlations, independent of the type of external field, is thereby achieved. The mapping is explicated for the example of a quantum dot in a magnetostatic field, and for a quantum dot in a magnetostatic and time-dependent electric field. Full article
Figures

Graphical abstract

Open AccessArticle Highly Excited States from a Time Independent Density Functional Method
Computation 2016, 4(3), 28; https://doi.org/10.3390/computation4030028
Received: 31 May 2016 / Revised: 29 July 2016 / Accepted: 1 August 2016 / Published: 5 August 2016
Cited by 2 | PDF Full-text (237 KB) | HTML Full-text | XML Full-text
Abstract
A constrained optimized effective potential (COEP) methodology proposed earlier by us for singly low-lying excited states is extended to highly excited states having the same spatial and spin symmetry. Basic tenets of time independent density functional theory and its COEP implementation for excited
[...] Read more.
A constrained optimized effective potential (COEP) methodology proposed earlier by us for singly low-lying excited states is extended to highly excited states having the same spatial and spin symmetry. Basic tenets of time independent density functional theory and its COEP implementation for excited states are briefly reviewed. The amended Kohn–Sham-like equations for excited state orbitals and their specific features for highly excited states are discussed. The accuracy of the method is demonstrated using exchange-only calculations for highly excited states of the He and Li atoms. Full article
Open AccessArticle Interaction of Hydrogen with Au Modified by Pd and Rh in View of Electrochemical Applications
Computation 2016, 4(3), 26; https://doi.org/10.3390/computation4030026
Received: 6 June 2016 / Revised: 7 July 2016 / Accepted: 13 July 2016 / Published: 20 July 2016
Cited by 4 | PDF Full-text (2323 KB) | HTML Full-text | XML Full-text
Abstract
Hydrogen interaction with bimetallic Au(Pd) and Au(Rh) systems are studied with the density functional theory (DFT)-based periodic approach. Several bimetallic configurations with varying concentrations of Pd and Rh atoms in the under layer of a gold surface(111) were considered. The reactivity of the
[...] Read more.
Hydrogen interaction with bimetallic Au(Pd) and Au(Rh) systems are studied with the density functional theory (DFT)-based periodic approach. Several bimetallic configurations with varying concentrations of Pd and Rh atoms in the under layer of a gold surface(111) were considered. The reactivity of the doped Au(111) toward hydrogen adsorption and absorption was related to the property modifications induced by the presence of metal dopants. DFT-computed quantities, such as the energy stability, the inter-atomic and inter-slab binding energies between gold and dopants, and the charge density were used to infer the similarities and differences between both Pd and Rh dopants in these model alloys. The hydrogen penetration into the surface is favored in the bimetallic slab configurations. The underlayer dopants affect the reactivity of the surface gold toward hydrogen adsorption in the systems with a dopant underlayer, covered by absorbed hydrogen up to a monolayer. This indicates a possibility to tune the gold surface properties of bimetallic electrodes by modulating the degree of hydrogen coverage of the inner dopant layer(s). Full article
Figures

Graphical abstract

Open AccessArticle Predictions of Physicochemical Properties of Ionic Liquids with DFT
Computation 2016, 4(3), 25; https://doi.org/10.3390/computation4030025
Received: 31 May 2016 / Revised: 28 June 2016 / Accepted: 13 July 2016 / Published: 19 July 2016
Cited by 7 | PDF Full-text (1380 KB) | HTML Full-text | XML Full-text
Abstract
Nowadays, density functional theory (DFT)-based high-throughput computational approach is becoming more efficient and, thus, attractive for finding advanced materials for electrochemical applications. In this work, we illustrate how theoretical models, computational methods, and informatics techniques can be put together to form a simple
[...] Read more.
Nowadays, density functional theory (DFT)-based high-throughput computational approach is becoming more efficient and, thus, attractive for finding advanced materials for electrochemical applications. In this work, we illustrate how theoretical models, computational methods, and informatics techniques can be put together to form a simple DFT-based throughput computational workflow for predicting physicochemical properties of room-temperature ionic liquids. The developed workflow has been used for screening a set of 48 ionic pairs and for analyzing the gathered data. The predicted relative electrochemical stabilities, ionic charges and dynamic properties of the investigated ionic liquids are discussed in the light of their potential practical applications. Full article
Figures

Graphical abstract

Open AccessArticle Orbital Energy-Based Reaction Analysis of SN2 Reactions
Computation 2016, 4(3), 23; https://doi.org/10.3390/computation4030023
Received: 1 June 2016 / Revised: 28 June 2016 / Accepted: 29 June 2016 / Published: 8 July 2016
Cited by 3 | PDF Full-text (1663 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
An orbital energy-based reaction analysis theory is presented as an extension of the orbital-based conceptual density functional theory. In the orbital energy-based theory, the orbitals contributing to reactions are interpreted to be valence orbitals giving the largest orbital energy variation from reactants to
[...] Read more.
An orbital energy-based reaction analysis theory is presented as an extension of the orbital-based conceptual density functional theory. In the orbital energy-based theory, the orbitals contributing to reactions are interpreted to be valence orbitals giving the largest orbital energy variation from reactants to products. Reactions are taken to be electron transfer-driven when they provide small variations for the gaps between the contributing occupied and unoccupied orbital energies on the intrinsic reaction coordinates in the initial processes. The orbital energy-based theory is then applied to the calculations of several S N2 reactions. Using a reaction path search method, the Cl + CH3I → ClCH3 + I reaction, for which another reaction path called “roundabout path” is proposed, is found to have a precursor process similar to the roundabout path just before this SN2 reaction process. The orbital energy-based theory indicates that this precursor process is obviously driven by structural change, while the successor SN2 reaction proceeds through electron transfer between the contributing orbitals. Comparing the calculated results of the SN2 reactions in gas phase and in aqueous solution shows that the contributing orbitals significantly depend on solvent effects and these orbitals can be correctly determined by this theory. Full article
Figures

Graphical abstract

Open AccessArticle On the v-Representabilty Problem in Density Functional Theory: Application to Non-Interacting Systems
Computation 2016, 4(3), 24; https://doi.org/10.3390/computation4030024
Received: 1 June 2016 / Revised: 28 June 2016 / Accepted: 29 June 2016 / Published: 5 July 2016
Cited by 3 | PDF Full-text (4770 KB) | HTML Full-text | XML Full-text
Abstract
Based on a computational procedure for determining the functional derivative with respect to the density of any antisymmetric N-particle wave function for a non-interacting system that leads to the density, we devise a test as to whether or not a wave function
[...] Read more.
Based on a computational procedure for determining the functional derivative with respect to the density of any antisymmetric N-particle wave function for a non-interacting system that leads to the density, we devise a test as to whether or not a wave function known to lead to a given density corresponds to a solution of a Schrödinger equation for some potential. We examine explicitly the case of non-interacting systems described by Slater determinants. Numerical examples for the cases of a one-dimensional square-well potential with infinite walls and the harmonic oscillator potential illustrate the formalism. Full article
Figures

Figure 1

Open AccessArticle On the Use of Benchmarks for Multiple Properties
Computation 2016, 4(2), 20; https://doi.org/10.3390/computation4020020
Received: 24 March 2016 / Revised: 20 April 2016 / Accepted: 20 April 2016 / Published: 30 April 2016
Cited by 4 | PDF Full-text (308 KB) | HTML Full-text | XML Full-text
Abstract
Benchmark calculations provide a large amount of information that can be very useful in assessing the performance of density functional approximations, and for choosing the one to use. In order to condense the information some indicators are provided. However, these indicators might be
[...] Read more.
Benchmark calculations provide a large amount of information that can be very useful in assessing the performance of density functional approximations, and for choosing the one to use. In order to condense the information some indicators are provided. However, these indicators might be insufficient and a more careful analysis is needed, as shown by some examples from an existing data set for cubic crystals. Full article
Figures

Figure 1

Open AccessArticle Kinetic and Exchange Energy Densities near the Nucleus
Computation 2016, 4(2), 19; https://doi.org/10.3390/computation4020019
Received: 15 February 2016 / Revised: 23 March 2016 / Accepted: 28 March 2016 / Published: 2 April 2016
Cited by 8 | PDF Full-text (371 KB) | HTML Full-text | XML Full-text
Abstract
We investigate the behavior of the kinetic and the exchange energy densities near the nuclear cusp of atomic systems. Considering hydrogenic orbitals, we derive analytical expressions near the nucleus, for single shells, as well as in the semiclassical limit of large non-relativistic neutral
[...] Read more.
We investigate the behavior of the kinetic and the exchange energy densities near the nuclear cusp of atomic systems. Considering hydrogenic orbitals, we derive analytical expressions near the nucleus, for single shells, as well as in the semiclassical limit of large non-relativistic neutral atoms. We show that a model based on the helium iso-electronic series is very accurate, as also confirmed by numerical calculations on real atoms up to two thousands electrons. Based on this model, we propose non-local density-dependent ingredients that are suitable for the description of the kinetic and exchange energy densities in the region close to the nucleus. These non-local ingredients are invariant under the uniform scaling of the density, and they can be used in the construction of non-local exchange-correlation and kinetic functionals. Full article
Figures

Figure 1

Open AccessArticle Current Issues in Finite-T Density-Functional Theory and Warm-Correlated Matter †
Computation 2016, 4(2), 16; https://doi.org/10.3390/computation4020016
Received: 18 February 2016 / Revised: 14 March 2016 / Accepted: 16 March 2016 / Published: 28 March 2016
Cited by 14 | PDF Full-text (322 KB) | HTML Full-text | XML Full-text
Abstract
Finite-temperature density functional theory (DFT) has become of topical interest, partly due to the increasing ability to create novel states of warm-correlated matter (WCM).Warm-dense matter (WDM), ultra-fast matter (UFM), and high-energy density matter (HEDM) may all be regarded as subclasses of WCM. Strong
[...] Read more.
Finite-temperature density functional theory (DFT) has become of topical interest, partly due to the increasing ability to create novel states of warm-correlated matter (WCM).Warm-dense matter (WDM), ultra-fast matter (UFM), and high-energy density matter (HEDM) may all be regarded as subclasses of WCM. Strong electron-electron, ion-ion and electron-ion correlation effects and partial degeneracies are found in these systems where the electron temperature Te is comparable to the electron Fermi energy EF. Thus, many electrons are in continuum states which are partially occupied. The ion subsystem may be solid, liquid or plasma, with many states of ionization with ionic charge Zj. Quasi-equilibria with the ion temperature Ti Te are common. The ion subsystem in WCM can no longer be treated as a passive “external potential”, as is customary in T = 0 DFT dominated by solid-state theory or quantum chemistry. Many basic questions arise in trying to implement DFT for WCM. Hohenberg-Kohn-Mermin theory can be adapted for treating these systems if suitable finite-T exchange-correlation (XC) functionals can be constructed. They are functionals of both the one-body electron density ne and the one-body ion densities ρj. Here, j counts many species of nuclei or charge states. A method of approximately but accurately mapping the quantum electrons to a classical Coulomb gas enables one to treat electron-ion systems entirely classically at any temperature and arbitrary spin polarization, using exchange-correlation effects calculated in situ, directly from the pair-distribution functions. This eliminates the need for any XC-functionals. This classical map has been used to calculate the equation of state of WDM systems, and construct a finite-T XC functional that is found to be in close agreement with recent quantum path-integral simulation data. In this review, current developments and concerns in finite-T DFT, especially in the context of non-relativistic warm-dense matter and ultra-fast matter will be presented. Full article
Figures

Figure 1

Open AccessArticle Influence of the Localization of Ge Atoms within the Si(001)(4 × 2) Surface Layer on Semicore One-Electron States
Computation 2016, 4(1), 14; https://doi.org/10.3390/computation4010014
Received: 24 December 2015 / Revised: 18 February 2016 / Accepted: 23 February 2016 / Published: 3 March 2016
Cited by 1 | PDF Full-text (1428 KB) | HTML Full-text | XML Full-text
Abstract
Adsorption complexes of germanium on the reconstructed Si(001)(4 × 2) surface have been simulated by the Si96Ge2Н84 cluster. For Ge atoms located on the surface layer, DFT calculations (B3LYP/6-31G**) of their 3d semicore-level energies have shown a clear-cut
[...] Read more.
Adsorption complexes of germanium on the reconstructed Si(001)(4 × 2) surface have been simulated by the Si96Ge2Н84 cluster. For Ge atoms located on the surface layer, DFT calculations (B3LYP/6-31G**) of their 3d semicore-level energies have shown a clear-cut correlation between the 3d5/2 chemical shifts and mutual arrangement of Ge atoms. Such a shift is positive when only one Ge atom penetrates into the crystalline substrate, while being negative for both penetrating Ge atoms. We interpret these results in terms of the charge distribution in clusters under consideration. Full article
Figures

Figure 1

Open AccessArticle Localized Polycentric Orbital Basis Set for Quantum Monte Carlo Calculations Derived from the Decomposition of Kohn-Sham Optimized Orbitals
Computation 2016, 4(1), 10; https://doi.org/10.3390/computation4010010
Received: 10 December 2015 / Accepted: 28 January 2016 / Published: 6 February 2016
Cited by 3 | PDF Full-text (561 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
In this work, we present a simple decomposition scheme of the Kohn-Sham optimized orbitals which is able to provide a reduced basis set, made of localized polycentric orbitals, specifically designed for Quantum Monte Carlo. The decomposition follows a standard Density functional theory (DFT)
[...] Read more.
In this work, we present a simple decomposition scheme of the Kohn-Sham optimized orbitals which is able to provide a reduced basis set, made of localized polycentric orbitals, specifically designed for Quantum Monte Carlo. The decomposition follows a standard Density functional theory (DFT) calculation and is based on atomic connectivity and shell structure. The new orbitals are used to construct a compact correlated wave function of the Slater–Jastrow form which is optimized at the Variational Monte Carlo level and then used as the trial wave function for a final Diffusion Monte Carlo accurate energy calculation. We are able, in this way, to capture the basic information on the real system brought by the Kohn-Sham orbitals and use it for the calculation of the ground state energy within a strictly variational method. Here, we show test calculations performed on some small selected systems to assess the validity of the proposed approach in a molecular fragmentation, in the calculation of a barrier height of a chemical reaction and in the determination of intermolecular potentials. The final Diffusion Monte Carlo energies are in very good agreement with the best literature data within chemical accuracy. Full article
Figures

Figure 1

Open AccessArticle Optical Properties of Silicon-Rich Silicon Nitride (SixNyHz) from First Principles
Computation 2015, 3(4), 657-669; https://doi.org/10.3390/computation3040657
Received: 14 October 2015 / Revised: 19 November 2015 / Accepted: 2 December 2015 / Published: 8 December 2015
Cited by 4 | PDF Full-text (1710 KB) | HTML Full-text | XML Full-text
Abstract
The real and imaginary parts of the complex refractive index of SixNyHz have been calculated from first principles. Optical spectra for reflectivity, absorption coefficient, energy-loss function (ELF), and refractive index were obtained. The results for Si3N
[...] Read more.
The real and imaginary parts of the complex refractive index of SixNyHz have been calculated from first principles. Optical spectra for reflectivity, absorption coefficient, energy-loss function (ELF), and refractive index were obtained. The results for Si3N4 are in agreement with the available theoretical and experimental results. To understand the electron energy loss mechanism in Si-rich silicon nitride, the influence of the Si/N ratio, the positions of the access Si atoms, and H in and on the surface of the ELF have been investigated. It has been found that all defects, such as dangling bonds in the bulk and surfaces, increase the intensity of the ELF in the low energy range (below 10 eV). H in the bulk and on the surface has a healing effect, which can reduce the intensity of the loss peaks by saturating the dangling bonds. Electronic structure analysis has confirmed the origin of the changes in the ELF. It has demonstrated that the changes in ELF are not only affected by the composition but also by the microstructures of the materials. The results can be used to tailor the optical properties, in this case the ELF of Si-rich Si3N4, which is essential for secondary electron emission applications. Full article
Figures

Figure 1

Open AccessArticle Assessment of Density-Functional Tight-Binding Ionization Potentials and Electron Affinities of Molecules of Interest for Organic Solar Cells Against First-Principles GW Calculations
Computation 2015, 3(4), 616-656; https://doi.org/10.3390/computation3040616
Received: 14 August 2015 / Revised: 31 October 2015 / Accepted: 19 November 2015 / Published: 4 December 2015
Cited by 8 | PDF Full-text (3678 KB) | HTML Full-text | XML Full-text
Abstract
Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (Voc) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and
[...] Read more.
Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (Voc) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and the 3ob parameter set (the third-order DFTB (DFTB3) organic and biochemistry parameter set) against experiments (for smaller molecules) and against first-principles GW (Green’s function, G, times the screened potential, W) calculations (for larger molecules of interest in organic electronics) for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT), but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ΔSCF (Δ self-consistent field) SCC-DFTB calculations for first IPs (good to ± 0.649 eV). When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans’ theorem (KT) IPs) as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ΔSCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here) for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ΔSCF calculations and of ± 0.326/0.458 eV from (KT) orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15–20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of interest to other researchers interested in DFTB investigation of large and complex problems, such as those encountered in organic electronics. Full article
Figures

Figure 1

Back to Top