The magnetic hyperfine field and electric-field gradient at isolated lanthanide impurities in an Fe host lattice are calculated from first principles, allowing for the first time a qualitative and quantitative understanding of an experimental data set collected over the past 40 years. It is demonstrated that the common Local Density Approximation leads to quantitatively and qualitatively wrong results, while the LDA+U method performs much better. In order to avoid pitfalls inherent to the LDA+U method, a careful strategy had to be used, which will be described in detail. The lanthanide 4f spin moment is found to couple antiferromagnetically to the magnetization of the Fe lattice, in agreement with the model of Campbell and Brooks. There is strong evidence for a delocalization/localization transition that is shifted from Ce to at least Pr and maybe further up to Sm. This shift is interpreted in terms of the effective pressure felt by lanthanides in Fe. Implications for resolving ambiguities in the determination of delocalization in pure lanthanide metals under pressure are discussed. For the localized lanthanides, Yb is shown to be divalent in this host lattice, while all others are trivalent (including Eu, the case of Tm is undecided). The completely filled and well-bound 5p shell of the lanthanides is shown to have a major and unexpected influence on the dipolar hyperfine field and on the electric-field gradient, a feature that can be explained by their 1/r^3 dependence. An extrapolation to actinides suggests that the same is true for the actinide 6p shell. The case of free lanthanide atoms is discussed as well.Comment: 17 pages, 14 figure

It is well known that present versions of density functional theory do not predict the experimentally observed spin-density wave state to be the ground state of Cr. Recently, a so-called "nodon model" has been proposed as an alternative way to reconcile theory and experiment: the ground state of Cr is truly antiferromagnetic, and the spin-density wave appears due to low-lying thermal excitations ͑"nodons"͒. We examine in this paper whether the postulated properties of these nodons are reproduced by ab initio calculations.

For some years already, ab initio calculations based on Density Functional Theory (DFT) belong to the toolbox of the field of hyperfine interaction studies. In this paper, the standard ab initio approach is schematically sketched. New features, methods and possibilities that broke through during the past few years are listed, and their relation to the standard approach is explained. All this is illustrated by some highlights of recent ab initio work done by the Nuclear Condensed Matter Group at the K.U.Leuven.

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