The Ejs Duffing Oscillator model computes the solutions to the non-linear Duffing equation, which reads, x'' + 2γx' - x (1 - x²) = f cos(ωt), where each prime denotes a time derivative. The simulation displays the solution as well as the phase space and Poincare plots, and energy diagram. The evolution parameters can be changed via textboxes. You can modify this simulation if you have Ejs installed by right-clicking within the plot and selecting “Open Ejs Model” from the pop-up menu item. Ejs Duffing Oscillator model was created using the Easy Java Simulations (Ejs) modeling tool. It is distributed as a ready-to-run (compiled) Java archive. Double clicking the ejs_ehu_chaos_Duffing.jar file will run the program if Java is installed. Ejs is a part of the Open Source Physics Project and is designed to make it easier to access, modify, and generate computer models. Additional Ejs models for non-linear dynamics and chaos are available. They can be found by searching ComPADRE for Open Source Physics, OSP, or Ejs.