  It was recently demonstrated that the Stillinger-Weber silicon undergoes a
liquid-liquid first-order phase transition deep into the supercooled region
(Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects
of perturbations on this phase transition. We show that the order of the
liquid-liquid transition changes with negative pressure. We also find that the
liquid-liquid transition disappears when the three-body term of the potential
is strengthened by as little as 5 %. This implies that the details of the
potential could affect strongly the nature and even the existence of the
liquid-liquid phase.
