# [lammps-users] Velocity

Hellow, everybody~~

I wonder how the velocity of atom is calculated in LAMMPS?

I tried to calculate the velocity of adatom which is deposited on the surface by using coodinate(z) of the adatom and elasped time.

However, my calculation results were different from the velocity calculated from LAMMPS.

2008/8/29 이순근 <soongun@…784…>

Hellow, everybody~~

I wonder how the velocity of atom is calculated in LAMMPS?

I tried to calculate the velocity of adatom which is deposited on the surface by using coodinate(z) of the adatom and elasped time.

However, my calculation results were different from the velocity calculated from LAMMPS.

By default, LAMMPS uses a velocity Verlet algorithm. You can find a lot of information about it in books like Allen and Tildesley, and from various papers.

Also, you’re confusing average velocity (distance over time) with instantaneous velocity (what LAMMPS needs).

–AEI

Assuming you are comparing average velocity (distance travelled over time) with the final instantaneous velocity, the two will only agree exactly if the particle velocity is constant in time. I suppose this might be a good approximation for a particle hitting the surface at high speed (until it actually hit the surface). If you are seeing orders of magnitude differences in the two velocities, then it many be because they are not in the same units (although generally, t, x, and v reported by LAMMPS are mutually consistent. See http://lammps.sandia.gov/doc/units.html.

2008/8/29 Ahmed E. Ismail <aeismail@…24…>