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.

Partition density functional theory is a formally exact procedure for calculating molecular properties from Kohn-Sham calculations on isolated fragments, interacting via a global partition potential that is a functional of the fragment densities. An example is given and consequences discussed. PACS numbers:Kohn-Sham density functional theory (KS-DFT) [1,2] is an efficient and usefully accurate electronic structure method, because it replaces the interacting Schrödinger equation with a set of single-particle orbital equations. Calculations with several hundred atoms are now routine, but there is always interest in much larger systems. Many such systems are treated by a lower-level method, such as molecular mechanics, but a fragment in which a chemical reaction occurs must still be treated quantum mechanically. A plethora of such QM/MM approaches have been tried and tested, with varying degrees of success [3]. These are often combined with attempts at orbital-free DFT, which avoids the KS equations, but at the cost of higher error and unreliability.On the other hand, partition theory (PT) [4,5] combines the simplicity of functional minimization with a density optimization to define fragments (such as atoms) within molecules, overcoming limitations of earlier approaches to reactivity theory [6,7]. While there are now many definitions of, e.g., charges on atoms, none have the generality of PT and the associated promise of unifying disparate chemical concepts. However, previous work on PT has been either formal [4,5] or for two atom systems [8,9].In this paper, we unite KS-DFT with PT to produce an algorithm that allows a KS calculation for a molecule to be performed via a self-consistent loop over isolated fragments. Such a fragment calculation exactly reproduces the result of a standard KS calculation of the entire molecule. We demonstrate its convergence on a 12-atom example. This also shows that fragments can be calculated 'on the fly', as part of solving any KS molecular problem.Thus we present a formally exact framework within which existing practical approximations can be analyzed and, for smaller systems, compared with exact quantities. In practical terms, our method suggests new approximations that can, by construction, scale linearly[10] with the number of fragments (so-called O(N )), and allow embedding of KS calculations within cruder force-field calculations (QM/MM). It also suggests ways to improve XC approximations so as to produce correct dissociation of molecules [11].To understand the relation between DFT and PT, recall that the Hohenberg-Kohn theorem proves that for a given electron-electron interaction and statistics, the external (one-body) potential v(r) is a unique functional of the density n(r). The total energy can be written as:where F [n] is a universal functional, defined by the LevyLieb constrained search [12] over all antisymmetric wavefunctions Ψ yielding density n(r):whereT andV ee are the kinetic energy and Coulomb repulsion operators respectively. The KS equations are single-particle...

The adiabatic approximation in time-dependent density functional theory (TDDFT) yields reliable excitation spectra with great efficiency in many cases, but fundamentally fails for states of double excitation character. We discuss how double-excitations are at the root of some of the most challenging problems for TDDFT today. We then present new results for (i) the calculation of autoionizing resonances in the helium atom, (ii) understanding the nature of the double excitations appearing in the quadratic response function, and (iii) retrieving double-excitations through a real-time semiclassical approach to correlation in a model quantum dot

We show that the exact exchange-correlation potential of time-dependent density-functional theory displays dynamical step structures that have a spatially non-local and time non-local dependence on the density. Using one-dimensional two-electron model systems, we illustrate these steps for a range of non-equilibrium dynamical situations relevant for modeling of photo-chemical/physical processes: field-free evolution of a non-stationary state, resonant local excitation, resonant complete charge-transfer, and evolution under an arbitrary field. Lack of these steps in usual approximations yield inaccurate dynamics, for example predicting faster dynamics and incomplete charge transfer.The vast majority of applications of time-dependent density functional theory (TDDFT) today deal with the calculation of the linear electronic spectra and response of molecules and solids, and provide an unprecedented balance between accuracy and efficiency [1,2]. The theorems of TDDFT also apply to any real-time electron dynamics, not necessarily starting in a ground-state, and possibly subject to strong or weak time-dependent fields. Time-resolved dynamics are particularly important and topical for TDDFT for two reasons. First, there is really no alternative practical method for accurately describing correlated electron dynamics, and second, many fascinating new phenomena and technological applications lie in this realm. These include: attosecond control of electron dynamics [3], photo-induced coupled electron-ion dynamics (for example in describing lightharvesting and artificial photosyntheses), and photochemical/physical processes [4,5] in general. TDDFT in theory yields all observables exactly, solely in terms of the time-dependent density, however in practice, approximations must be made both for the observable as a functional of the density, and for the exchangecorrelation (xc) functional. Thus the question arises as to whether the approximate functionals that have been successful for excitations predict equally well the dynamics in the more general time-dependent context. In particular, the exact xc contribution to the Kohn-Sham (KS) potential at time t functionally depends on the history of the density n(r, t ′ < t), the initial interacting many-body state Ψ 0 , and the choice of the initial KS state Φ 0 : v XC [n; Ψ 0 , Φ 0 ](r, t). However, almost all calculations today use an adiabatic approximation, v A XC = v g.s. XC