# How does Lammps calculate Property/atom fx fy fz of an atom using ReaxFF

Hello all,
As far as I know from Lammps documentation "ReaxFF uses distance-dependent bond-order functions to represent the contributions of chemical bonding to the potential energy. " Now, I was wondering whether differentiating distance-dependent bond energy between any two atoms with respect to radius of separation between them can be perceived as force induced by one on another. If that is true does lammps use some weighing summation to calculate the total force on an atom which we get to know by command “Property/atom fx fy fz of an atom using ReaxFF”. Thanks a lot.

Yours Truly
Anubhav Roy
University of Alabama
Department of Aerospace Engineering and Mechanics
Tuscaloosa, U.S.

Hello all,
As far as I know from Lammps documentation "ReaxFF uses
distance-dependent bond-order functions to represent the contributions of
chemical bonding to the potential energy. " Now, I was wondering whether
differentiating distance-dependent bond energy between any two atoms with
respect to radius of separation between them can be perceived as *force*
induced by one on another. If that is true does lammps use some weighing
summation to calculate the total force on an atom which we get to know by
command "Property/atom fx fy fz of an atom using ReaxFF". Thanks a lot.

​by looking at the change in energy due to the distance between two
selected atoms, you can do a projection of the full potential energy
function on that distance. however, since ReaxFF is not a pairwise additive
potential - unlike, say, lennard-jones or morse - this projection will
change when the distances of these two atoms to other atoms changes.

there is no need of "weighting". the ReaxFF force field implementation
simply computes the individual terms of the force field, most of which - as
explained above - depend on more complex information than just the distance
between two particles, and then sums the contribution into the per-atom
force arrays. for the details of the potential functions employed, please
have a look at the papers cited in the documentation of the reax and reax/c
pair styles.

axel.

Hello,
Thanks a lot for the detailed explanations. I had a look at the original papers cited in the documentation. However I would see if I get any clue to get force between two atoms any way as a part of post-processing. Thanks a lot again.