# Homework Help: Calculating atomic velocities from forces

1. Jul 2, 2012

### Hypatio

1. The problem statement, all variables and given/known data

I am trying to use interatomic pair potentials to calculate forces from velocities, but I seem to arrive at ridiculous values.

2. Relevant equations (and 3. Solution attempt)

Consider that I have a potential function where (r is distance, E is potential energy, Ang is Angstroms)

dE/dr @ 3 Ang is about 0.0342 eV/Ang, or 3.42x10^8 eV/m, or in Joules: 5.48x10^-21 J/m.

Using the mass of oxygen, 1.34x10^-26 kg, and classical mechanics (a=F/m) this force should apply an acceleration of 4.1x10^15 m/s^2.

My calculated acceleration suggests that the velocity should change by on the order of meters per femtosecond, at least 10 orders of magnitude greater than what would make molecular dynamics simulations viable.

Have I made a mistake?

2. Jul 3, 2012

### Simon Bridge

You seem to have confused your units ... E has units of volts per length ... so dE/dr would be volts per area?
Don't know why you are doing dE/dr anyway - don't you want something more like: E=dV/dr ?
F=qE ... what did you use for q?

I cannot figure what mistake you have made, though, unless I see your actual calculation.