I want to give a external field for the system witten as
V(x_i).

This potential is "not" pairwise potential, but 1-body force
which acts on a certain atom type.
For example, V(x_i)=V_0 2\sin(2\pi x_i/L) where L is system size.

Could you tell me how to write the input script
and give me the sample command ?

I want to give a external field for the system witten as
V(x_i).

This potential is "not" pairwise potential, but 1-body force
which acts on a certain atom type.
For example, V(x_i)=V_0 2\sin(2\pi x_i/L) where L is system size.

Could you tell me how to write the input script
and give me the sample command ?

to the best of my knowledge you cannot do with with LAMMPS
without a little bit of programming. however, this programming
would be fairly simple: you best implement this as a new "fix"
command. see for example the "fix efield" which implements a
homogeneous electric field with an arbitrary direction. fixes
act on groups and allow you to manipulate atomic properties
including forces in very flexible ways.

cheers,
axel.

p.s.: please send my regards to wataru and ikeshoji-sama