  To study the possibility of a fluid-fluid phase transition, we analyze a
three-dimensional soft-core isotropic potential for a one-component system. We
utilize two independent numerical approaches, (i) integral equation in the
hypernetted-chain approximation and (ii) molecular dynamics simulations, and
find for both approaches a fluid-fluid phase transition as well as the
conventional gas-liquid critical point. We also study the possible existence of
a triple point in the supercooled fluid phase at which three phases---gas,
high-density fluid, and low-density fluid---coexist.
