  The structural properties of the simulated $\rm Cu_{\alpha}Zr_{1-\alpha}$
glassy alloys are studied in the wide range of the copper concentration
$\alpha$ to clarify the impact of the composition on the number density of the
icosahedral clusters. Both bond orientational order parameters and Voronoi
tessellation methods are used to identify these clusters. Our analysis shows
that abundance of the icosahedral clusters and the chemical composition of
these clusters are essentially non monotonic versus $\alpha$ and demonstrate
local extrema. That qualitatively explains the existence of pinpoint
compositions of high glass-forming ability observing in Cu-Zr alloys. Finally,
it has been shown that Voronoi method overestimates drastically the abundance
of the icosahedral clusters in comparison with the bond orientational order
parameters one.
