When droplets are placed on hydrophobic textured surfaces, different wetting states Cassie Baxter (CB) state or Wenzel (W) state may occur depending on materials and structures of surfaces, types and sizes of droplets, thermal fluctuations, and external stimuli. The wetting transition from the CB to the W state and the opposite process have attracted a great deal of attention because of their primary importance for designing and fabricating textured surfaces. In this work, molecular dynamics (MD) simulations are employed to understand the mechanism behind the CB-to-W transition for a nanoscale water film placed on a surface decorated with a single nanogroove when an external electric field is applied. The free energy variation during the transition process is computed on the basis of the restrained MD simulations. Water intrusion into the groove is observed by simulation snapshots, which provides direct evidence for the electric field induced CB-to-W transition. In the previous experiments, however, only a sharp reduction in the apparent contact angle is employed to judge whether the transition takes place. The free energy curves reveal that there are two energy barriers separating the CB and W states (Delta E-1) as well as separating the W and CB states (Delta E-2). Owing to the presence of Delta E-1 although the CB state has a higher free energy than the W state, it cannot spontaneously convert to the W state. When the external energy input exceeds Delta E-1 the CB-to-W transition can be triggered, otherwise the transition will stop, and the water film will return to the CB state. Moreover, it is found that the maximum of free energy always occurs after the film touches the groove bottom. Thus, the requirement that the film should touch the groove bottom is responsible for the presence of the energy barrier Delta E-1. Finally, the dependence of the two energy barriers on the electric field strength, groove aspect ratio, and intrinsic contact angle of the groove is also discussed.