Long-range excitations in time-dependent density functional theory

Adiabatic time-dependent density functional theory fails for excitations of a heteroatomic molecule composed of two open-shell fragments at large separation. Strong frequency-dependence of the exchange-correlation kernel is necessary for both local and charge-transfer excitations. The root of this is static correlation created by the step in the exact Kohn-Sham ground-state potential between the two fragments. An approximate non-empirical kernel is derived for excited molecular dissociation curves at large separation. Our result is also relevant for the usual local and semi-local approximations for the ground-state potential, as static correlation there arises from the coalescence of the highest occupied and lowest unoccupied orbital energies as the molecule dissociates.Comment: 7 pages, 2 figure

Continuum states from time-dependent density functional theory

Linear response time-dependent density functional theory is used to study low-lying electronic continuum states of targets that can bind an extra electron. Exact formulas to extract scattering amplitudes from the susceptibility are derived in one dimension. A single-pole approximation for scattering phase shifts in three dimensions is shown to be more accurate than static exchange for singlet electron-He$^+$ scattering.Comment: 5 pages, 2 figures, J. Chem. Phys. accepte

Magnetic properties of doped GdI2

Motivated by the recent experimental studies on layered ferromagnetic metallic system GdI2 and its doped variant GdI2Hx we develop a model to understand their ground state magnetic phase diagram. Based on first principle electronic structure calculations we write down a phenomenological model and solve it under certain approximations to obtain the ground state energy. In the process we work out the phase diagram of the correlated double exchange model on a triangular lattice for the specific band structure at hand.Comment: 13 pages, 5 figures, corrected typo

Time-dependent density functional theory: Past, present, and future

Time-dependent density functional theory (TDDFT) is presently enjoying enormous popularity in quantum chemistry, as a useful tool for extracting electronic excited state energies. This article discusses how TDDFT is much broader in scope, and yields predictions for many more properties. We discuss some of the challenges involved in making accurate predictions for these properties.Comment: 12 pages, 4 figure

On the density-potential mapping in time-dependent density functional theory

The key questions of uniqueness and existence in time-dependent density functional theory are usually formulated only for potentials and densities that are analytic in time. Simple examples, standard in quantum mechanics, lead however to non-analyticities. We reformulate these questions in terms of a non-linear Schr\"odinger equation with a potential that depends non-locally on the wavefunction.Comment: 8 pages, 2 figure

Interplay of magnetism and band topology in Eu1−x_{1-x}Cax_xMg2_2Bi2_2 (x=0, 0.5) from first principles study

Recent discovery of the time reversal symmetry breaking magnetic Weyl semimetals has created a huge surge of activities in the field of quantum topological materials. In this work, we have studied systematically the ground state magnetic order, electronic structure and the interplay between the magnetic order and band topology in one such materials, EuMg$_2$Bi$_2$ (EMB) and its Ca doped variant using first principles method within the framework of density functional theory (DFT). The detailed investigation unravels the existence of different topological phases in this single material which can be tuned by an external probe such as magnetic field or chemical substitution. Our DFT calculations including Coulomb correlation (U) and spin-orbit (SO) interaction within GGA+U+SO approximation confirms that the magnetic ground state of EMB is A-type Antiferromagnetic (A-AFM) with Eu magnetic moments aligned along the crystallographic $a$ or $b$ direction. Although the ground state of EMB is A-AFM, the Ferromagnetic (FM) state lies very close in energy. We observe a single pair of Weyl points connecting valence and conduction band very close to the Fermi level (FL) along $\Gamma$-A direction in the FM state of EuMg$_2$Bi$_2$ with Eu moments aligned along crystallographic $c$ direction. On doping 50\% Ca at Eu sites, we observe single pair of Weyl points moving closer to the FL which is highly desirable for application purposes. Further we observe that the separation between the Weyl points in the pair decreases in doped compound compared to that in the parent compound which has direct consequence on anomalous Hall conductivity (AHC). Our first principles calculation of AHC shows high peak values exactly at these Weyl points and the peak height decreases when we dope the system with Ca. Therefore, Ca doping can be a good external handle to tune AHC in this system.Comment: 9 pages, 12 figure

Semiclassical Electron Correlation in Density-Matrix Time-Propagation

Lack of memory (locality in time) is a major limitation of almost all present time-dependent density functional approximations. By using semiclassical dynamics to compute correlation effects within a density-matrix functional approach, we incorporate memory, including initial-state dependence, as well as changing occupation numbers, and predict more observables in strong-field applications.Comment: 4.5 pages, 1 figur