Converting Deposition Velocity to Desired Kinetic Energy (metal units)

LAMMPS
1

Hello LAMMPS users,

I am currently simulating the deposition of carbon atoms onto a silicon substrate using units metal in LAMMPS.

The deposition command I am using is:

lammps

CopyEdit

region     	slab block 0 5 0 5 5 8 units lattice
fix         5 addatoms deposit 2000 2 25 12345 region slab near 3.0 vx 0 0 vy 0 0 vz -30.0 -30.0

To estimate the incident kinetic energy (KE) of the depositing carbon atoms, I am using the standard formula:

KE=12mv2\text{KE} = \frac{1}{2} m v^2KE=21​mv2

where:

  • mmm is the atomic mass in amu (for carbon, 12.0),
  • vvv is the velocity in Å/ps,
  • and the result is expected in eV since I’m using metal units.

My questions are:

  1. To compute the kinetic energy in eV, do I need to convert the mass and velocity to SI units first, or does this formula work directly under metal units?
  2. If I want to deposit carbon atoms with incident kinetic energies ranging from 10 eV to 100 eV, what would be the appropriate velocity values I should assign in the fix deposit command?

Any clarification or advice would be greatly appreciated.

Thank you!

2

Hi @Amjad,

This is a basic physics question not related to LAMMPS use. Please, refer to any class or course material related to dimensional analysis.

If you want a quick and dirty workaround, you can make a simulation with a single carbon with some initial velocity and look at the kinetic energy. Else you’ll have to do the math.

3

Note that Google is pretty good at interpreting natural language units for this kind of conversion. For example, it tells me that 1 amu * (1 angstrom / picosecond)^2 is about 0.1 meV: 1 amu * (1 angstrom/picosecond)^2 in eV - Google Search