finding local stress and surface tension

The fix ave/spatial method will give the correct total surface
tension. Local stress is more tricky but the fix ave/spatial method should
be fairly close. If you want a more exact answer you can try Hardy's
approach (or the Irving-Kirkwood method). Depending on which
component of the pressure tensor you need, you could also try the
method of planes (MOP) which wouldn't be too
hard to implement in LAMMPS. It can't give you all the components of
the pressure though.