  We present calculations of electronic and magnetic structures of
vanadium-benzene multidecker clusters V$_{n}$Bz$_{n+1}$ ($n$ = 1 - 3) using
advanced quantum Monte Carlo methods. These and related systems have been
identified as prospective spin filters in spintronic applications, assuming
that their ground states are half-metallic ferromagnets. Although we find that
magnetic properties of these multideckers are consistent with ferromagnetic
coupling, their electronic structures do not appear to be half-metallic as
previously assumed. In fact, they are ferromagnetic insulators with large and
broadly similar $\uparrow$-/$\downarrow$-spin gaps. This makes the potential of
these and related materials as spin filtering devices very limited, unless they
are further modified or functionalized.
