  We perform molecular dynamics simulations of a well-known water model (the
TIP5P pair potential) and a simple liquid model (a two-scale repulsive ramp
potential) to compare the regions of anomalous behavior in their phase
diagrams. We select the parameters of the ramp potential by mapping it to an
effective pair potential derived from the TIP5P model. We find that the regions
of anomalous behavior in the phase diagrams of both systems can be mapped onto
each other if (i) pressure $P$ and temperature $T$ are replaced by $T-T_{C}$
and $P-P_{C}$, respectively, where $(T_{C},P_{C})$ are the coordinates of the
liquid-liquid critical point of the corresponding system; and (ii) a single
ramp particle corresponds to two TIP5P molecules. We present heuristic
arguments supporting point (ii). We also argue that the water-like anomalies in
the ramp potential are due to the ability of the particles to reproduce, upon
compression or heating, the migration of water molecules from the second shell
to its first shell.
