# How can I exert stress boundary correctly?

Hello, all

I am a new user to lammps.
After read the manual, I have learned there are several ways to
exert boundary condition. Now I want to simulate a box
and exert stress boundary on some faces (because of some reasons,
I think prescribed displacement on these faces isn't appropriate.).

I plan to add a force on each atom of the top three layers of each face
by the command "fix addforce". The force exerted on each atom is
obtained by dividing the total force I want to exert to this face by
the amount of atom of the top three layers.

But I'm not sure is it a correct way to exert boundary condition?

Wang Kai

You can use fix addforce on some subset of atoms. Or fix
move to move them in a prescribed fashion. Or you can
use fix deform to stretch the box itself, for example.

Steve

Dear Steve,

Thank you very much for your help!

Actually, what I want to know is if I can calculate
the total force by multiplying the force exerted on
each atom and the amount of atoms?

For example, I exert a "fix addforce 0.1 0.0 0.0" on each
atom of the top three layers of a face, and the amount of
atoms is 662. Then, can I speak I exert a x-axis force
0.1*662=66.2 on this face? Is it a appropriate way to exert
boundary condition? (For some reasons, prescribed displacement
isn't suit for my case.)

Wang Kai

yes, you can sum the force across atoms.

Steve