  We present a study of the electronic and magnetic properties of the
multiple-decker sandwich nanowires ($CP-M$) composed of cyclopentadienyl (CP)
rings and 3d transition metal atoms (M=Ti to Ni) using first-principles
techniques. We demonstrate using Density Functional Theory that structural
relaxation play an important role in determining the magnetic ground-state of
the system. Notably, the computed magnetic moment is zero in $CP-Mn$, while in
$CP-V$ a significant turn-up in magnetic moment is evidenced. Two compounds
show a half-metallic ferromagnetic ground state $CP-Fe/Cr$ with a gap within
minority/majority spin channel. In order to study the effect of electronic
correlations upon the half-metallic ground states in $CP-Cr$, we introduce a
simplified three-bands Hubbard model which is solved within the Variational
Cluster Approach. We discuss the results as a function of size of the reference
cluster and the strength of average Coulomb $U$ and exchange $J$ parameters.
Our results demonstrate that for the range of studied parameters $U=2-4eV$ and
$J=0.6-1.2eV$ the half-metallic character is not maintained in the presence of
local Coulomb interactions.
