# Apply multiple pairs of force

Dear all,

I am trying to apply forces between atom pairs. For example, the coordinate of atom A(x1,y1,z1), atom B(x2,y2,z2). The direction of force on atom A is (x2-x1,y2-y1,z2-z1), and on atom B (x1-x2,y1-y2,z1-z2), and has constant value F. i.e. there is always a pair of force having the same absolute value between A and B and point to each other.

I use “fix addforce” command to do this, so I have to define 2 groups to track the coordinate of A and B. Now I want to add force on atom pair A-B, C-D, E-F… However, since LAMMPS has a limit on the number of group, I can only add no more than 16 pairs of force.

So if I want to add more pairs of force, is there any alternative method to add force like this? Or I do have to write new “fix” function?

Thank you and best wishes,

Wen

Dear all,

I am trying to apply forces between atom pairs. For example, the coordinate
of atom A(x1,y1,z1), atom B(x2,y2,z2). The direction of force on atom A is
(x2-x1,y2-y1,z2-z1), and on atom B (x1-x2,y1-y2,z1-z2), and has constant
value F. i.e. there is always a pair of force having the same absolute value
between A and B and point to each other.

I use "fix addforce" command to do this, so I have to define 2 groups to
track the coordinate of A and B. Now I want to add force on atom pair A-B,
C-D, E-F... However, since LAMMPS has a limit on the number of group, I can
only add no more than 16 pairs of force.

...and this is for a good reason! if there would be more groups and
you would apply more of these, you would have an *extremely* slow and
inefficient simulation.
also, you have to keep in mind, that a computation like this would
only work for an isolated system.

So if I want to add more pairs of force, is there any alternative method to
add force like this? Or I do have to write new "fix" function?

the probably most efficient and straightforward way to add this
feature would be to modify pair style list to include a constant
force.

axel.