  We compare the percolation loci for chemical clusters with the liquid-solid
transition in the temperature-density phase diagram. Chemical clusters are
defined as sets of particles connected through particle-particle bonds that
last for a given time tau. By using molecular dynamics simulations of a
Lennard-Jones system we obtain the percolation loci at different values of tau
as the lines in the temperature-density plane at which the system presents a
spanning cluster in 50 percent of the configurations. We find that the
percolation loci for chemical clusters shifts rapidly towards high densities as
tau is increased. For moderate values of tau this line coincides with the
low-density branch of the liquid-solid coexistence curve. This implies that no
stable chemical clusters can be found in the fluid phase. In contrast, the
percolation loci for physical clusters -sets of particles that remain close
together at every instant for a given period tau- tends to a limiting line, as
tau tends to infinity, which is far from the liquid-solid transition line.
