You can always post your question here (this forum). I need to have your ejs source code in order to know your problem better and help you more! Please send email to hwang@phy,ntnu.edu.tw and attach you ejs source. The best ODE solver is Runge-Kutta 4th orrder methos. There are several similar example under Simulations subdirectory. Please check out those examples. If you have several arrow, please use arrowSet instead of arrow for each object. Fu-Kwun