Recently , type Ia supernovae data appear to support a dark energy whose equation of state w crosses -1 , which is a much more amazing problem than the acceleration of the universe .
We show that it is possible for the equation of state to cross the phantom divide by a scalar field in the gravity with an additional inverse power-law term of Ricci scalar in the Lagrangian .
The necessary and sufficient condition for a universe in which the dark energy can cross the phantom divide is obtained .
Some analytical solutions with w < -1 or w > -1 are obtained .
A minimal coupled scalar with different potentials , including quadratic , cubic , quantic , exponential and logarithmic potentials are investigated via numerical methods , respectively .
All these potentials lead to the crossing behavior .
We show that it is a robust result which is hardly dependent on the concrete form of the potential of the scalar .