  The electronic structure of the rational approximants 1/1 Cd6Yb and 1/1 Cd6Ca
to the stable icosahedral CdYb and CdCa quasicrystals is studied by the
full-potential linear augmented plane wave method. A comparison is made between
several structural models. We show that the (relativistic) spin-orbit (SO)
interaction and electronic correlations that are not described by the usual
local density approximation, are essential for an accurate description of the
electronic structure. In particular, we show that the SO interaction is
responsible for a splitting of the Cd-4d and Yb-4f peaks, and that the
experimental peak positions can be reproduced by including a Hubbard U term in
the Hamiltonian [U(Cd) = 5.6 eV, U(Yb) = 3.1 eV]. Our results show very good
agreement with a photo-emission (PE) spectrum of 1/1 Cd6Yb [R. Tamura, Y.
Murao, S. Takeuchi, T. Kiss, T. Yokoya, and S. Shin, Phys. Rev. B 65, 224207
(2002)] and a 350 eV PE spectrum of 1/1 Cd6Ca, which we present in this paper.
Without the relativistic and correlation effects even a qualitative agreement
with the PE spectra cannot be achieved.
