  The ability of parameter recently proposed by Sorensen et al. (Nature, 409,
63 (2001)) to characterize entanglement or inseparability is investigated for a
system of two two-level atoms interacting with a single mode of radiation
field. For comparison we employ the necessary and sufficient criterion of Peres
and Horodecki for inseparability of bipartite states. We show that for diagonal
atomic density matrix the parameter of Sorensen et al. fails to show entangled
nature of collective atomic state whereas same state is inseparable in
accordance with Peres-Horodecki criterion.
