Lagrangian formalism in perturbed nonlinear Klein–Gordon equations

We develop an alternative approach to study the effect of the generic perturbation (in addition to explicitly considering the loss term) in the nonlinear Klein–Gordon equations. By a change of the variables that cancel the dissipation term we are able to write the Lagrangian density and then, calculate the Lagrangian as a function of collective variables. We use the Lagrangian formalism together with the Rice Ansatz to derive the equations of motion of the collective coordinates (CCs) for the perturbed sine-Gordon (sG) and ϕ 4 systems. For the N collective coordinates, regardless of the Ansatz used, we show that, for the nonlinear Klein–Gordon equations, this approach is equivalent to the Generalized Traveling Wave Ansatz (GTWA).