excitation energies
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2022 ◽  
Vol 2022 ◽  
pp. 1-6
Author(s):  
Chang-Youn Moon ◽  
Kee-Suk Hong ◽  
Yong-Sung Kim

We investigate defect properties in hexagonal boron nitride (hBN) which is attracting much attention as a single photon emitter. Using first-principles calculations, we find that nitrogen-vacancy defect V N has a lower energy structure in C 1 h symmetry in 1− charge state than the previously known D 3 h symmetry structure. Noting that carbon has one more valence electron than boron species, our finding naturally points to the correspondence between V N and V N C B defects with one charge state difference between them, which is indeed confirmed by the similarity of atomic symmetries, density of states, and excitation energies. Since V N C B is considered as a promising candidate for the source of single photon emission, our study suggests V N as another important candidate worth attention, with its simpler form without the incorporation of foreign elements into the host material.


2022 ◽  
Author(s):  
Tim Gould ◽  
Zahed Hashimi ◽  
Leeor Kronik ◽  
Stephen Dale

In calculations based on density functional theory, the "HOMO-LUMO gap" (difference between the highest occupied and lowest unoccupied molecular orbital energies) is often used as a low-cost, ad hoc approximation for the lowest excitation energy. Here we show that a simple correction based on rigorous ensemble density functional theory makes the HOMO-LUMO gap exact, in principle, and significantly more accurate, in practice. The introduced perturbative ensemble density functional theory approach predicts different and useful values for singlet-singlet and singlet-triplet excitations, using semi-local and hybrid approximations. Excitation energies are of similar quality to time-dependent density functional theory, especially at high fractions of exact exchange. It therefore offers an easy-to-implement and low-cost route to robust prediction of molecular excitation energies.


2022 ◽  
Author(s):  
Hui-Hui Zhang ◽  
Hua-Lei Wang ◽  
Hai-Yan Meng ◽  
Min-Liang Liu ◽  
Bing Ding

Abstract Rotational structure properties along the yrast line for 766 observed even-even nuclei with Z ≥ 20 in the nuclide chart have been systematically studied by means of the approach of pairing-deformation self-consistent total Routhian surface calculations in three-dimensional deformation space (β 2, γ, β 4). Typical two-dimensional maps of the total nuclear energy are presented as functions of rotational frequency ̄hω. Various types of physical quantities (including nuclear shapes, aligned angular momenta, pairing gaps and excitation energies) are presented in the (Z, N) plane, indicating the overall characteristics. The ground-state deformations are compared with experimental data and other theoretical results. The present investigation shows that the Coriolis coupling may affect the overall properties systematically, for instance, enforcing regular drifts of the different deformation ‘islands’. We believe that the synthetic presentation will be helpful when planning high-spin experiments, especially in the data-scarce drip-line or superheavy regions. Moreover, such systematic and large-scale calculation and analysis can help overcoming and eliminating the bias among different theoretical models and be useful for checking and developing them.


Author(s):  
Zachary Jordan ◽  
Shahriar N. Khan ◽  
Benjamin A. Jackson ◽  
Evangelos Miliordos

Abstract Density functional theory and ab initio multi-reference calculations are performed to examine the stability and electronic structure of boron complexes that host diffuse electrons in their periphery. Such complexes (solvated electron precursors or SEPs) have been experimentally identified and studied theoretically for several s- and d-block metals. For the first time, we demonstrate that a p-block metalloid element can form a stable SEP when appropriate ligands are chosen. We show that three ammonia and one methyl ligands can displace two of the three boron valence electrons to a peripheral 1s-type orbital. The shell model for these outer electrons is identical to previous SEP systems (1s, 1p, 1d, 2s). Further, we preformed the first examination of a molecular system consisting of two SEPs bridged by a hydrocarbon chain. The electronic structure of these dimers is very similar to that of traditional diatomic molecules forming bonding and anti-bonding σ and π orbitals. Their ground state electronic structure resembles that of two He atoms, and our results indicate that the excitation energies are nearly independent of the chain length for four carbon atoms or longer. These findings pave the way for the development of novel materials similar to expanded metals and electrides.


2022 ◽  
Vol 23 (2) ◽  
pp. 621
Author(s):  
Marine Lebel ◽  
Thibaut Very ◽  
Eric Gloaguen ◽  
Benjamin Tardivel ◽  
Michel Mons ◽  
...  

The present benchmark calculations testify to the validity of time-dependent density functional theory (TD-DFT) when exploring the low-lying excited states potential energy surfaces of models of phenylalanine protein chains. Among three functionals suitable for systems exhibiting charge-transfer excited states, LC-ωPBE, CAM-B3LYP, and ωB97X-D, which were tested on a reference peptide system, we selected the ωB97X-D functional, which gave the best results compared to the approximate coupled-cluster singles and doubles (CC2) method. A quantitative agreement for both the geometrical parameters and the vibrational frequencies was obtained for the lowest singlet excited state (a ππ* state) of the series of capped peptides. In contrast, only a qualitative agreement was met for the corresponding adiabatic zero-point vibrational energy (ZPVE)-corrected excitation energies. Two composite protocols combining CC2 and DFT/TD-DFT methods were then developed to improve these calculations. Both protocols substantially reduced the error compared to CC2 and experiment, and the best of both even led to results of CC2 quality at a lower cost, thus providing a reliable alternative to this method for very large systems.


2022 ◽  
Vol 105 (1) ◽  
Author(s):  
E. M. Kozulin ◽  
G. N. Knyazheva ◽  
I. M. Itkis ◽  
M. G. Itkis ◽  
Y. S. Mukhamejanov ◽  
...  
Keyword(s):  

RSC Advances ◽  
2022 ◽  
Vol 12 (3) ◽  
pp. 1704-1717
Author(s):  
Qabas Alkhatib ◽  
Wissam Helal ◽  
Ali Marashdeh

The excitation energies of 13 BODIPY dye sensitizers are benchmarked by means of TD-DFT, using 36 functionals. Spin-component-scaled double-hybrid (DSD) functionals are found to show the best performance.


2021 ◽  
Author(s):  
Astrid Tarleton ◽  
Jorge Garcia-Alvarez ◽  
Anah Wynn ◽  
Cade Awbrey ◽  
Tomas Roberts ◽  
...  

Excited-state quantum chemical calculations typically report excitation energies and oscillator strengths, ƒ, for each electronic transition. On the other hand, UV-visible spectrophotometric experiments report energy-dependent molar extinction/attenuation coefficients, ε(v), that determine the absorption band line shapes. ε(v) and ƒ are related, but this relation is complicated by various broadening and solvation effects. We fit and integrated experimental UV-visible spectra to obtain ƒexp values for absorption bands and estimated the uncertainty in the fitting. We derived 164 ƒexp values from 100 organic molecules ranging in size from 6-34 atoms. The corresponding computed oscillator strengths (ƒcomp) were obtained with time-dependent density functional theory and a polarizable continuum solvent model. By expressing experimental and computed absorption strengths using a common quantity, we directly compared ƒcomp and ƒexp. While ƒcomp and ƒexp are well correlated (linear regression R2=0. 921), ƒcomp in most cases significantly overestimates ƒexp (regression slope=1.34). The agreement between absolute ƒcomp and ƒexp values was substantially improved by accounting for a solvent refractive index factor, as suggested in some derivations in the literature. The 100 digitized UV-visible spectra are included as plain text files in the supporting information to aid in benchmarking computational or machine-learning approaches that aim to simulate realistic UV-visible absorption spectra.


2021 ◽  
Vol 9 ◽  
Author(s):  
Alexander Yakubovich ◽  
Alexey Odinokov ◽  
Sergey Nikolenko ◽  
Yongsik Jung ◽  
Hyeonho Choi

We present a computational workflow based on quantum chemical calculations and generative models based on deep neural networks for the discovery of novel materials. We apply the developed workflow to search for molecules suitable for the fusion of triplet-triplet excitations (triplet-triplet fusion, TTF) in blue OLED devices. By applying generative machine learning models, we have been able to pinpoint the most promising regions of the chemical space for further exploration. Another neural network based on graph convolutions was trained to predict excitation energies; with this network, we estimate the alignment of energy levels and filter molecules before running time-consuming quantum chemical calculations. We present a comprehensive computational evaluation of several generative models, choosing a modification of the Junction Tree VAE (JT-VAE) as the best one in this application. The proposed approach can be useful for computer-aided design of materials with energy level alignment favorable for efficient energy transfer, triplet harvesting, and exciton fusion processes, which are crucial for the development of the next generation OLED materials.


2021 ◽  
Vol 9 ◽  
Author(s):  
Chenfei Shen ◽  
Xianwei Wang ◽  
Xiao He

Fluorescent RNA aptamers have been successfully applied to track and tag RNA in a biological system. However, it is still challenging to predict the excited-state properties of the RNA aptamer–fluorophore complex with the traditional electronic structure methods due to expensive computational costs. In this study, an accurate and efficient fragmentation quantum mechanical (QM) approach of the electrostatically embedded generalized molecular fractionation with conjugate caps (EE-GMFCC) scheme was applied for calculations of excited-state properties of the RNA aptamer–fluorophore complex. In this method, the excited-state properties were first calculated with one-body fragment quantum mechanics/molecular mechanics (QM/MM) calculation (the excited-state properties of the fluorophore) and then corrected with a series of two-body fragment QM calculations for accounting for the QM effects from the RNA on the excited-state properties of the fluorophore. The performance of the EE-GMFCC on prediction of the absolute excitation energies, the corresponding transition electric dipole moment (TEDM), and atomic forces at both the TD-HF and TD-DFT levels was tested using the Mango-II RNA aptamer system as a model system. The results demonstrate that the calculated excited-state properties by EE-GMFCC are in excellent agreement with the traditional full-system time-dependent ab initio calculations. Moreover, the EE-GMFCC method is capable of providing an accurate prediction of the relative conformational excited-state energies for different configurations of the Mango-II RNA aptamer system extracted from the molecular dynamics (MD) simulations. The fragmentation method further provides a straightforward approach to decompose the excitation energy contribution per ribonucleotide around the fluorophore and then reveals the influence of the local chemical environment on the fluorophore. The applications of EE-GMFCC in calculations of excitation energies for other RNA aptamer–fluorophore complexes demonstrate that the EE-GMFCC method is a general approach for accurate and efficient calculations of excited-state properties of fluorescent RNAs.


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