$w_d=-1$ in interacting quintessence model
  A model consisting of quintessence scalar field interacting with cold dark
matter is considered. Conditions required to reach $w_d=-1$ are discussed. It
is shown that depending on the potential considered for the quintessence,
reaching the phantom divide line puts some constraints on the interaction
between dark energy and dark matter. This also may determine the ratio of dark
matter to dark energy density at $w_d=-1$.
