High energy collisions

To anyone who can help,

I am running a simulation of high-energy PKAs (>20keV). I notice, initially, the energy of the PKA decreases below 20 keV once the PKA hits the surface. But afterwards, I noticed that the PKA might gain a little energy (as a much as 100 eV over the course of 100 time steps). The energy slowly fluctuates downwards, but I am having a hard time understanding how a 20 keV ion could gain energy from a bulk sample that is at 300 K.

Could this be something that makes physical sense, or is this most likely an error in the setup of the simulation?

Jackson

Jackson,

How did you create the PKA and what did you mean by the “surface”? How did you monitor the energy of the PKA? What is the bulk sample?

Ray

i created the PKA through the deposit command. I think these changes are just fluctuations and I think it might make sense. No matter the material I use or the force field, I always see this slight fluctuation. I monitor the energy by using the compute ke/atom command and then I just follow the energy of the PKA.

The energy does slowly go down, I just notice that it also increases sometimes from a given timestep to another, but the general trend is downward.

To anyone who can help,

I am running a simulation of high-energy PKAs (>20keV). I notice, initially,
the energy of the PKA decreases below 20 keV once the PKA hits the surface.
But afterwards, I noticed that the PKA might gain a little energy (as a much
as 100 eV over the course of 100 time steps). The energy slowly fluctuates
downwards, but I am having a hard time understanding how a 20 keV ion could
gain energy from a bulk sample that is at 300 K.

have you considered what happens, when atoms collide at high speed -
especially when they are perfectly on top of each other - and your MD
time step is (too?) large?

axel.

Sorry but that is not considered as a PKA – primary knock-on atom, but a high incident energy *bombardment. What you saw is quite normal in these bombardment simulations.

Ray

  1. are you using a variable time step?
  2. is the interatomic potential you use stiffened? I.e most of the EAM and other potentials are created for equilibrium simulations and not for high energy PKA impacts where the atoms can come very close to each other. At these distances (<< 1 Å) the potential fitting is not good. The pair part of some EAM potentials may even be attractive at these sort distances. Therefore you must use a stiffened potential for high energy PKA simulations.

Best regards
Manoj

Yes, I am using variable time step. Yes I am using ZBL force field. I will take the advice of Dr. Shan that this is normal to see.

Thanks,

Jackson