Fourth-order gradient corrections to the exchange-only energy functional: Importance of

Tuning optoelectronic response of lateral core-alloyed crown nanoplatelets: type-II CdSe-CdSe1-xTex

We present a theoretical study showing that the exciton emission in the CdSe-CdSe_1-xTe_xcore-alloyed crown heterostructure results from the tunable quasi-type II to pure type II behavior by adjusting the Te to Se ratio. We suggest that the direct crown exciton or interface indirect exciton or a dual emission can be tuned due to the altered conduction band offset. We also found that these different emissions are red-shifted with increasing the nanoplatelets (NPLs) monolayer (ML) thickness due to the quantum confinement effect. The double exciton emission develops caused by the band bowing effect occurring in the alloyed crown. The band bowing is originated from the difference between the bonding nature of the Se and Te orbitals with the Cd orbitals in the conduction band edge states. We also found that the band bowing is sensitive on the alloyed-crown ML thickness and the in-plane strain due to hybridization magnitude between the cation (Cd) and anion (Se, Te). Our results are in accord with the available experimental data. We propose the CdSe-CdSe_1-xTe_xcore-alloyed crown NPLs as a promising contender for the near-infrared-emitting heterostructures preparation used for light-harvesting applications.

Anion-Cation Replacement Effect on the Structural and Optoelectronic Properties of the LiMX2 (M = Al, Ga, In; X = S, Se, Te) Compounds: A First Principles Study

In the chalcopyrite (or tetragonal) phase, different physical properties of the ternary LiMX2 (M = Al, Ga, In and X = S, Se, Te) compounds are studied by the very accurate density functional method. The optimized lattice constants and the bandgaps are close to the existing experimental data. In addition, for most of the LiMX2 compounds, when the cations change from Al to In and anions from S to Te, the lattice constant and equilibrium volume for the crystal unit cell increase whereas the bulk modulus decreases. Using different generalized gradient approximations, the band structure calculations are performed. Generally, it was observed that there exists a decreasing tendency of the bandgap energies except for the LiAlSe2, LiInSe2, and LiGaTe2 compounds due to the change from Al to In as well as the change from S to Te. The bonding analysis shows that ionic bonds are present between the Li-X atoms, while a covalent bond exists between the M cations and X anions. The optical properties of the compounds are studied by calculating the real and imaginary components of the refractive index, reflectivity, optical conductivity, and birefringence. In addition, the optical properties from the calculations show that these materials are appropriate applicants to be utilized as Bragg’s reflector or applied in optoelectronic and solar cell technology.

General Performance of Density Functionals

The density functional theory (DFT) foundations date from the 1920s with the work of Thomas and Fermi, but it was after the work of Hohenberg, Kohn, and Sham in the 1960s, and particularly with the appearance of the B3LYP functional in the early 1990s, that the widespread application of DFT has become a reality. DFT is less computationally demanding than other computational methods with a similar accuracy, being able to include electron correlation in the calculations at a fraction of time of post-Hartree-Fock methodologies. In this review we provide a brief outline of the density functional theory and of the historic development of the field, focusing later on the several types of density functionals currently available, and finishing with a detailed analysis of the performance of DFT across a wide range of chemical properties and system types, reviewed from the most recent benchmarking studies, which encompass several well-established density functionals together with the most recent efforts in the field. Globally, an overall picture of the level of performance of the plethora of currently available density functionals for each chemical property is drawn, with particular attention being dedicated to the relative performance of the popular B3LYP density functional.

The optimized effective potential with finite temperature

The optimized effective potential (OEP) method provides an additional level of exactness in the computation of electronic structures, e.g. the exact exchange energy can be used. This extra freedom is likely to be important in moving density functional methods beyond traditional approximations such as the local density approximation. We provide a new density-matrix-based derivation of the gradient of the Kohn-Sham energy with respect to the effective potential. This gradient can be used to iteratively minimize the energy in order to find the OEP. Previous work has indicated how this can be done in the zero temperature limit. This paper generalizes the previous results to the finite temperature regime. Equating our gradient to zero gives a finite temperature version of the OEP equation.

Gradient-corrected density-functional potential with correct asymptotic behavior: Application to interconfigurational energies in transition-metal atoms

Based upon the optimized effective potential with the self-interaction correction, we present in this paper an alternative gradient-corrected density-functional approximation with the proper long-range behavior of the effective potential. As applied to the study of the interconfigurational energies of the whole transition-metal atoms, the present combination of the gradient-corrected contribution and the modified optimized effective potential lead the s ionization to the excellent agreement with the experiment. The calculated d ionizations and s-d transition energies are also discussed.

Simple iterative construction of the optimized effective potential for orbital functionals, including exact exchange

For exchange-correlation functionals that depend explicitly on the Kohn-Sham orbitals, the potential V(xcsigma)(r) must be obtained as the solution of the optimized effective potential (OEP) integral equation. This is very demanding and has limited the use of orbital functionals. We demonstrate that instead the OEP can be obtained iteratively by solving the partial differential equations for the orbital shifts that exactify the Krieger-Li-Iafrate approximation. Unoccupied orbitals do not need to be calculated. Accuracy and efficiency of the method are shown for atoms and clusters using the exact-exchange energy. Counterintuitive asymptotic limits of the exact OEP are presented.

Quantum Monte Carlo analysis of exchange and correlation in the strongly inhomogeneous electron gas

We use the variational quantum Monte Carlo method to calculate the density-functional exchange-correlation hole n(xc), the exchange-correlation energy density e(xc), and the total exchange-correlation energy E(xc) of several strongly inhomogeneous electron gas systems. We compare our results with the local density approximation and the generalized gradient approximation. It is found that the nonlocal contributions to e(xc) contain an energetically significant component, the magnitude, shape, and sign of which are controlled by the Laplacian of the electron density.