Comment on "Soliton ratchets induced by excitation of internal modes"
  Very recently Willis et al. [Phys. Rev. E {\bf 69}, 056612 (2004)] have used a collective variable theory to explain the appearance of a nonzero energy current in an ac driven, damped sine-Gordon equation. In this comment, we prove rigorously that the time-averaged energy current in an ac driven nonlinear Klein-Gordon system is strictly zero. 
Niurka R. Quintero, Bernardo S\'anchez-Rey, Jes\'us Casado-Pascual