X-ray absorption spectroscopy (XAS) is widely employed for structure characterization of graphitic carbon nitride (g-C$_3$N$_4$) and its composites. Nevertheless, even for pure g-C$_3$N$_4$, discrepancies in energy and profile exist across different experiments, which can be attributed to variations in structures arising from diverse synthesis conditions and calibration procedures. Here, we conducted a theoretical investigation on XAS of three representative g-C$_3$N$_4$ structures (planar, corrugated, and micro-corrugated) optimized with different strategies, to understand the structure-spectroscopy relation. Different methods were compared, including density functional theory (DFT) with the full (FCH) or equivalent (ECH) core-hole approximation, as well as the time-dependent DFT (TDDFT). FCH was responsible for getting accurate absolute absorption energy; while ECH and TDDFT aided in interpreting the spectra, through ECH-state canonical molecular orbitals (ECH-CMOs) and natural transition orbitals (NTOs), respectively. With each method, the spectra at the three structures show evident differences, which can be correlated to different individual experiments or in between. Our calculations explained the structural reason behind the spectral discrepancies among different experiments. Moreover, profiles predicted by these methods also displayed consistency, so their differences can be used as a reliable indicator of their accuracy. Both ECH-CMOs and NTO particle orbitals led to similar graphics, validating their applicability in interpreting the transitions. This work provides a comprehensive analysis of the structure-XAS relation for g-C$_3$N$_4$, provides concrete explanations for the spectral differences reported in various experiments, and offers insight for future structure dynamical and transient X-ray spectral analyses.
A top-level designed forecasting system for predicting computational times of density-functional theory (DFT)/time-dependent density-functional theory (TDDFT) calculations is presented. The computational time is assumed as the intrinsic property for the molecule. Basing on this assumption, the forecasting system is established using the "reinforced concrete", which combines the cheminformatics, several machine-learning (ML) models, and the framework of many-world interpretation (MWI) in multiverse ansatz. Herein, the cheminformatics is used to recognize the topological structure of molecules, …
Imaginary-time time-dependent Density functional theory (it-TDDFT) has been proposed as an alternative method for obtaining the ground state within density functional theory (DFT) which avoids some of the difficulties with convergence encountered by the self-consistent-field (SCF) iterative method. It-TDDFT was previously applied to clusters of atoms where it was demonstrated to converge in select cases where SCF had difficulty with convergence. In the present work we implement it-TDDFT propagation for {\it periodic systems} by modifying the Quantum ESPRESSO package, which uses a plane-wave ba…
We report on time-dependent density functional theory (TDDFT) calculations of the excited states of 63 different graphene quantum dots (GQDs) in square shape with side lengths 1 nm, 1.5 nm and 2 nm. We investigate the systematics and trends in the UV-Vis absorption spectra of these GQDs, which are doped with elements B, N, O, S and P at dopant percentages 1.5%, 3%, 5% and 7%. The results show how the peaks in the UV and visible parts of the spectrum as well as the total absorption evolve in the chemical parameter space along the coordinates of size, dopant type and dopant percentage. The absor…
Time-dependent density functional theory (TDDFT) simulations of transient core-level spectroscopies require a balanced treatment of both valence- and core-electron excitations. To this end, tuned range-separated hybrid exchange-correlation functionals within the generalized Kohn-Sham scheme offer a computationally efficient means of simultaneously improving the accuracy of valence and core excitation energies in TDDFT by mitigating delocalization errors across multiple length-scales. In this work range-separated hybrid functionals are employed in conjunction with the velocity-gauge formulation…
Photo-active systems are characterized by their capacity of absorbing light energy and transforming it. Usually, more than one chromophore is involved in the light absorption and excitation transport processes in complex systems. Linear-Response Time-Dependent Density Functional (LR-TDDFT) is commonly used to identify excitation energies and transition properties by solving well-known Casida's equation for single molecules. However, this methodology is not useful in practice when dealing with multichromophore systems. In this work, we extend our local density decomposition method that enables …
When calculating the optical absorption spectra of molecular crystals from first principles, the influence of the crystalline environment on the excitations is of significant importance. For such systems, however, methods to describe the excitations accurately can be computationally prohibitive due to the relatively large system sizes involved. In this work, we demonstrate a method that allows optical absorption spectra to be computed both efficiently and at high accuracy. Our approach is based on the spectral warping method successfully applied to molecules in solvent. It involves calculating…
We present INQ, a new implementation of density functional theory (DFT) and time-dependent DFT (TDDFT) written from scratch to work on graphical processing units (GPUs). Besides GPU support, INQ makes use of modern code design features and takes advantage of newly available hardware. By designing the code around algorithms, rather than against specific implementations and numerical libraries, we aim to provide a concise and modular code. The result is a fairly complete DFT/TDDFT implementation in roughly 12,000 lines of open-source C++ code representing a modular platform for community-driven …
Time-dependent density functional theory (TDDFT) has been applied to the calculation of absorption spectra for two-dimensional atomic layer materials. We reveal that the character of the first bright exciton state of bi-layer hexagonal boron nitride (h-BN) is dependent on the layer stacking type through the use of many-body perturbation theory (MBPT) calculations, i.e., the electron and hole in the AA$'$ stacking are present in the same layer (intralayer exciton) while the A$'$B stacking exhibits an interlayer exciton. We demonstrate that the TDDFT approach with the meta-generalized gradient a…
Real-time time-dependent density functional theory (rt-TDDFT) with hybrid exchange-correlation functional has wide-ranging applications in chemistry and material science simulations. However, it can be thousands of times more expensive than a conventional ground state DFT simulation, hence is limited to small systems. In this paper, we accelerate hybrid functional rt-TDDFT calculations using the parallel transport gauge formalism, and the GPU implementation on Summit. Our implementation can efficiently scale to 786 GPUs for a large system with 1536 silicon atoms, and the wall clock time is onl…
Understanding, optimizing, and controlling the optical absorption process, exciton gemination, and electron-hole separation and conduction in low dimensional systems is a fundamental problem in materials science. However, robust and efficient methods capable of modelling the optical absorbance of low dimensional macromolecular systems and providing physical insight into the processes involved have remained elusive. We employ a highly efficient linear combination of atomic orbitals (LCAOs) representation of the Kohn--Sham (KS) orbitals within time dependent density functional theory (TDDFT) in …
Alloying effects on the optical properties of Ge$_{1-x}$Si$_x$ nanocrystals from TDDFT and comparison with effective-medium theory
采用含时密度泛函理论，在绝热局域密度近似（TDLDA）下，计算了Ge $_{1-x}$ Si $_x$合金纳米晶的光谱.光谱作为组成$x$的函数平滑地变化。在Ge侧的组成范围内，在吸收边的最低激发几乎是纯Kohn-Sham独立粒子HOMO-LUMO跃迁，而对于更高的Si含量，发现强烈的混合跃迁。然而，吸收的开始，特别是它的combustion依赖性是类似的独立粒子的结果。此外，经典的去极化效应是一个非常强的抑制的吸收强度负责。我们表明，他们可以考虑在一个更简单的方式使用麦克斯韦-加内特经典有效介质理论。发射光谱是通过计算激发态纳米晶体在其弛豫几何形状下的吸收来研究的。结构对斯托克斯位移的贡献约为0.5 eV。发射光谱在独立粒子渡越离子方面的分解与吸收的分解相似。对于发射，在富锗团簇中发现非常弱的跃迁，远低于强吸收起始。
Non-fullerene acceptors (NFAs) are excellent light harvesters, yet the origin of such high optical extinction is not well understood. In this work, we investigate the absorption strength of NFAs by building a database of time-dependent density functional theory (TDDFT) calculations of ~500 pi-conjugated molecules. The calculations are first validated by comparison with experimental measurements on liquid and solid state using common fullerene and non-fullerene acceptors. We find that the molar extinction coefficient (ε_(d,max)) shows reasonable agreement between calculation in vacuum and exper…
Wigner-Seitz truncated TDDFT approach for the calculation of exciton binding energies in solids
Centro de Física de Materiales - Materials Physics Center (CFM-MPC)（材料物理中心）；Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；EHU Quantum Center, Universidad del País Vasco/Euskal Herriko Unibertsitatea UPV/EHU（巴斯克大学量子中心）
The cesium copper halides CsCu$_2$X$_3$ (X=Cl, Br and I) are a class of all-inorganic perovskites with interesting and potentially useful optical properties, characterized by distinct excitonic features. We present a computational study of the optical absorption spectra of CsCu$_2$X$_3$, comparing time-dependent density-functional theory (TDDFT) and the Bethe-Salpeter equation (BSE), using $GW$ quasiparticle band structures as input. The TDDFT calculations are carried out using several types of global hybrid exchange-correlation functionals. It is found that an admixture of nonlocal exchange d…
Optical absorption spectra of metal oxides from time-dependent density functional theory and many-body perturbation theory based on optimally-tuned hybrid functionals
Explicit core-hole single-particle methods for L- and M- edge X-ray absorption and electron energy-loss spectra
FIT–Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg（弗赖堡大学 FIT-互动材料和技术仿生物理中心）；University of Vienna, Faculty of Physics（维也纳大学物理系）
Spectroscopies that probe electronic excitations from core levels into unoccupied orbitals, such as X-ray absorption spectroscopy and electron energy loss spectroscopy, are widely used to gain insight into the electronic and chemical structure of materials. To support the interpretation of experimental spectra, we assess the performance of a first-principles approach that combines linear-response time-dependent density (TDDFT) functional theory with the $Δ$~self\nobreakdash-consistent field ($Δ$SCF) approach. In particular, we first use TDDFT to calculate the core-level spectrum and then shift…
We have studied the nature of excitons in the transition metal dichalcogenide alloy Mo0.6W0.4 S2, compared to pure MoS2 and WS2 grown by atomic layer deposition (ALD). For this, optical absorption/transmission spectroscopy and time-dependent density functional theory (TDDFT) were used. Effects of temperature on the A and B exciton peak energies and linewidths in the optical transmission spectra were compared between the alloy and pure MoS2 and WS2. On increasing the temperature from 25 K to 293 K the energy of the A and B exciton peaks decreases, while their linewidth increases due to exciton-…
Orbital Transformers for Predicting Wavefunctions in Time-Dependent Density Functional Theory
Department of Computer Science and Engineering, Texas A&M University（计算机科学与工程系，德克萨斯大学阿马尔科分校）；Department of Materials Science and Engineering, Texas A&M University（材料科学与工程系，德克萨斯大学阿马尔科分校）；J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University（J. Mike Walker ’66 机械工程系，德克萨斯大学阿马尔科分校）；Department of Electrical and Computer Engineering, Texas A&M University（电气与计算机工程系，德克萨斯大学阿马尔科分校）；Department of Physics and Astronomy, Texas A&M University（物理与天文学系，德克萨斯大学阿马尔科分校）
It has been demonstrated in previous experimental and computational work that doping CeO2 with transition metals is an effective way of tuning its properties. However, each previous study on CeO2 doping has been limited to a single or a few dopants. In this paper, we systematically study the formation energies, structural stability and electronic properties of CeO2 doped with the entire range of the ten 3d transition metals using density functional theory (DFT) calculations at the hybrid level. The formation energies of oxygen vacancies, and their effects on electronic properties, were also co…
The electronic structure of high-quality van der Waals NiPS$_3$ crystals was studied using near-edge x-ray absorption spectroscopy (NEXAFS) and resonant photoelectron spectroscopy (ResPES) in combination with density functional theory (DFT) approach. The experimental spectroscopic methods, being element specific, allow to discriminate between atomic contributions in the valence and conduction band density of states and give direct comparison with the results of DFT calculations. Analysis of the NEXAFS and ResPES data allows to identify the NiPS$_3$ material as a charge-transfer insulator. Obta…
Density functional theory (DFT) based first-principles investigations of structural, elastic, electronic band structure, and optical properties of superconducting ternary phosphides (BaM2P2; M = Ni, Rh) have been carried out in this study. This is the first detailed pressure dependent study of these properties for the titled compounds. The calculated ambient condition properties are compared with existing experimental and theoretical results, where available. The pressure dependent variations of the electronic density of states at the Fermi level, N(EF), and the Debye temperature, thetaD, have…
