1. The problem statement, all variables and given/known data I am gonna use the velocity verlet algorithm to simulate the position, velocity and acceleration at time t of some particle. 2. Relevant equations We got the Langevin equation: a(t) = -v(t)/tau - U'(x) / m + F_f(t) / m Where tau is the mass divided by the friction coefficient, U'(x(t)) the external force and F_f(t) the fluctuating force. Let's say the external force is harmonic, ½kx(t)^2, so U'(x(t)) / m = kx(t)/m. I have so far typed in x(t+dt), v(t+dt/2) and I need to find what a(t+dt) is. It's easier if you go see http://en.wikipedia.org/wiki/Verlet_integration for the equations i refer to :) As for a(t+dt) I am in doubt how I should find this, what I tried so far is: a(t+dt) = -1/m * dV(x(t+dt))/dx = -v(t) / tau - k*x(t+dt) / m + F_f(t) (Where I take the value of the fluctuating force randomly from a normaldistribution) My doubt is about the use of v(t), can I use v(t) in the above equation? I don't see how many other options I have since we need a(t+dt) to be able to calculate v(t+dt), or have I missed something? I hope someone can help me a bit, if you need more info or anything is unclear, please tell me here.