group can straddle a periodic boundary.  See the "dump"_dump.html doc

page for a discussion of unwrapped coordinates.  It also means that a

spring connecting two groups or a group and the tether point can cross

a periodic boundary and its length be calculated correctly.  One

exception is for rigid bodies, which should not be used with the fix

spring command, if the rigid body will cross a periodic boundary.

This is because image flags for rigid bodies are used in a different

way, as explained on the "fix rigid"_fix_rigid.html doc page.

a periodic boundary and its length be calculated correctly.  

[Restart, fix_modify, output, run start/stop, minimize info:]