Because of a typing error, we wrote that the SBZ of the Cu͑100͒ surface is a square with a side of length 2.55 Å Ϫ1 . The correct value is 2.46 Å Ϫ1 . This error is irrelevant to the results.

The magnetic properties of Mn, Co and Ni substituted Fe4N are calculated from first principles theory. It is found that the generalized gradient approximation reproduces with good accuracy the magnetic moment and equilibrium volume for the parent Fe4N structure, with the atomic moment largest for the Fe atom furthest away from the N atom (Fe I site), approaching a value of 3 µB/atom, whereas the Fe atom closer to the N atom (Fe II site) has a moment closer to that of bcc Fe. Substitution of Fe for Mn, Co or Ni, shows an intricate behavior in which the Mn substitution clearly favors the Fe II site, Ni favors substitution on the Fe I site and Co shows no strong preference for either lattice site. Ni and Co substitution results in a ferromagnetic coupling to the Fe atoms, whereas Mn couples antiferromagnetically on the Fe II site and ferromagnetically on the Fe I site. For all types of doping, the total magnetic moment is enhanced compared with Fe4N only in the energetically very unfavorable case of Mn doping at the Fe I site.

We present the surface Reflectivity Anisotropy spectra of the relaxed (1x1) (110) surface of Cu and Ag as obtained by ab initio calculations. We are able to disentagle the effects of the intraband and interband parts of the bulk dielectric function on the bare dielectric anisotropy of the surface. We show how the position, sign and amplitude of the structures observed in such spectra depend on the above quantities. The lineshape of all the calculated structures agree very well with the ones observed experimentally for samples treated by suitable surface cleaning. In particular, we reproduce the observed single peak structure of Ag at high energy, found to represent a state of the clean surface different from the one giving the originally observed double peak structure. This results is not reproduced by the 'local field' model.

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