# LJ force direction

Hello Users,

I am pretty new to the molecular dynamics scenario and lammps. I have a very basic question in MD, I would
appreciate if anyone can help me.

Suppose there are two atoms in space each denoted by by vectors a and b for atoms a and b respectively.
Now suppose both the atoms are very near such that they are experiencing repulsive force due to LJ formulation.

Now force on a due to be will be f(a-b) and force of b acting on a will be f(b-a)= - f(a-b).

My question is if i have all the atoms in right handed coordinate system, the repulsive force of a on b
will be in one direction (assume +z) and force of b on a will be in other direction (-z) how to relate
these both? How to know which direction the force is acting? I know this is due to Newtons law but
if one wants the force acting on z direction for a group of atoms due to LJ formulation i can get different results
based on this sign convention How do i know the total force acting in a particular direction on a group of atoms?

One more question i have is how does the sign on force due to the repulsion/attraction relate to the sign of the axes?

Thanks,
Freddie

Hello Users,

I am pretty new to the molecular dynamics scenario and lammps. I have a very basic question in MD, I would
appreciate if anyone can help me.

Suppose there are two atoms in space each denoted by by vectors a and b for atoms a and b respectively.
Now suppose both the atoms are very near such that they are experiencing repulsive force due to LJ formulation.

Now force on a due to be will be f(a-b) and force of b acting on a will be f(b-a)= - f(a-b).

My question is if i have all the atoms in right handed coordinate system, the repulsive force of a on b
will be in one direction (assume +z) and force of b on a will be in other direction (-z) how to relate
these both? How to know which direction the force is acting? I know this is due to Newtons law but
if one wants the force acting on z direction for a group of atoms due to LJ formulation i can get different results
based on this sign convention How do i know the total force acting in a particular direction on a group of atoms?

One more question i have is how does the sign on force due to the repulsion/attraction relate to the sign of the axes?

handedness of the coordinate system has no impact on this.
both atoms repel each other. remember that those a point
particles and the force acts on the center of mass only.
that is it.

axel.

Dr.Axel,

Thanks for the reply. Just another question. If i calculate the total force on group1 atoms
due to group2 atoms using compute group1/group2 what will the sign on the total force imply?
I believe it just represents that the total force is acting in said direction of the coordinate axes.

am i right?

Thanks,
Freddie.

Dr.Axel,

Thanks for the reply. Just another question. If i calculate the total force on group1 atoms
due to group2 atoms using compute group1/group2 what will the sign on the total force imply?

it is the force of group2 on group1. if you swap the groups the sign of the force will change.

I believe it just represents that the total force is acting in said direction of the coordinate axes.

yes. but it is the force along the directional vector, which has a sign, hence the sign flip, if you swap the groups.

axel.

Dr. Axel,

Thanks for the help. I now understand how this works.

Regards,
Freddie.