  We present a comparative study of the glass forming ability of binary systems
with varying composition, where the systems have similar global crystalline
structure (CsCl+fcc). Biased Monte Carlo simulations using umbrella sampling
technique shows that the free energy cost to create a CsCl nucleus increases as
the composition of the smaller particles are decreased. We find that the
systems with comparatively lower free energy cost to form CsCl nucleus exhibit
more pronounced pre-crystalline demixing near the liquid/crystal interface. The
structural frustration between the CsCl and fcc crystal demands this demixing.
We show that closer to the equimolar mixture the entropic penalty for demixing
is lower and a glass forming system may crystallize spontaneously when seeded
with a nucleus. This entropic penalty as a function of composition shows a
non-monotonic behavior with a maximum at a composition similar to the well
known Kob-Anderson (KA) model. Although the KA model shows the maximum entropic
penalty and thus maximum frustration against CsCl formation, it also shows a
strong tendency towards crystallization into fcc lattice of the larger "A"
particles which can be explained from the study of the energetics. Thus for
systems closer to the equimolar mixture although it is the requirement of
demixing which provides their stability against crystallization, for KA model
it is not demixing but slow dynamics and structural frustration caused by the
locally favored structure around the smaller "B" particles which make it a good
glass former. Although the glass forming binary systems studied here are quite
similar, differing only in composition, we find that their glass forming
ability cannot be attributed to a single phenomena.
