The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements

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Time-dependent density functional theory (TDDFT) is implemented in an all electron solid-state code for the case of fully unconstrained noncollinear spins. We use this to study intense, short, laser pulse-induced demagnetization in bulk Fe, Co, Ni and find that demagnetization can take place on time scales of <20 fs. It is demonstrated that this form of demagnetization is a two-step process: excitation of a fraction of electrons followed by spin-flip transitions mediated by spin–orbit coupling of the remaining localized electrons. We further show that it is possible to control the moment loss by tunable laser parameters, including frequency, duration, and intensity.

A new parameter-free approximation for the exchange-correlation kernel fxc of time-dependent density functional theory is proposed. This kernel is expressed as an algorithm in which the exact Dyson equation for the response as well as a further approximate condition are solved together selfconsistently leading to a simple parameter-free kernel. We apply this to the calculation of optical spectra for various small bandgap (Ge, Si, GaAs, AlN, TiO2, SiC), large bandgap (C, LiF, Ar, Ne) and magnetic (NiO) insulators. The calculated spectra are in very good agreement with experiment for this diverse set of materials, highlighting the universal applicability of the new kernel. PACS numbers:The ab-initio calculation of optical absorption spectra of nano-structures and solids is a formidable task. The current state-of-the-art is based on many-body perturbation theory. A typical calculation involves two distinct steps: First, the quasi-particle spectral density function is calculated using the GW approximation, yielding accurate electron removal and addition energies, and therefore a good prediction for the fundamental gap [1]. In the second step, the Bethe-Salpeter equation (BSE) is solved using the one-body Green's function obtained in the GW step. Resonances, corresponding to bound electron-hole pairs called excitons, which have energies inside the gap, can then appear in the spectrum. The two step procedure described above is a well-established method for yielding macroscopic dielectric tensors which are generally in good agreement with experiment [2][3][4][5][6][7]. Unfortunately, solving the BSE involves diagonalizing a large matrix which couples different Bloch state k-points. As a consequence, the method is computationally expensive.Time-dependent density functional theory (TDDFT) [8], which extends density functional theory into the time domain, is another method able, in principle, to determine neutral excitations of a system. Although formally exact, the predictions of TDDFT are only as good as the approximation of the exchange-correlation (xc) kernel: f xc (r, r , t − t ) ≡ δv xc (r, t)/δρ(r , t ), where v xc is the TD exchange-correlation potential and ρ is the TD density. There are several such approximate kernels in existence, the earliest of which is the adiabatic local density approximation (ALDA) [9], where v xc (r, t) is determined from the usual ground-state local density approximation (LDA), calculated instantaneously for ρ(r, t). In practice however, the macroscopic dielectric function calculated using this kernel has two well-known deficiencies: the quasi-particle gap is too small and the physics of the bound electron-hole pair is totally missing -in fact ALDA does not improve on the results obtained within the random phase approximation (RPA) which corresponds to the trivial kernel f xc = 0 [10]. In the present work we concentrate on the second of these problems, namely the missing excitonic peak in the spectrum. There have been previous attempts to solve this problem [11], and there exist kernels...

Laser pulses induce spin-selective charge flow that we show to generate dramatic changes in the magnetic structure of materials, including a switching of magnetic order from antiferromagnetic (AFM) to transient ferromagnetic (FM) in multisub-lattice systems. The microscopic mechanism underpinning this ultrafast switching of magnetic order is dominated by spin-selective charge transfer from one magnetic sublattice to another. Because this spin modulation is purely optical in nature (i.e., not mediated indirectly via the spin-orbit interaction) this is one of the fastest means of manipulating spin by light. We further demonstrate this mechanism to be universally applicable to AFM, FM, and ferri-magnets in both multilayer and bulk geometry and provide three rules that encapsulate early-time magnetization dynamics of multisub-lattice systems.