How to understand the meaning of words in the blue box? How can a bond cross periodic boundaries and connect two atoms with the same image flag?
When the two atoms sit on opposite ends of the box.
But at this situation, the flag of the atom on the bottom of the box will be greater than that of the atom on the top by one.
No.
Consider this simple scheme which numbers represent the individual atoms ‘-’ a bond, and ‘|’ the box boundary:
|0-1-2-3-4-|(0)
The last bond would be from atom 4 to atom 0, but both 0 and 4 have the same image flag (say 0), but the consistent situation for this bond would be that the periodic image of ) (= (0)) which is the closest image for the 4-0 bond has an image flag of 1. If you would wrap atom 0 around, then this one bond would become consistent, but then the 0-1 bond would have inconsistent image flags.
Bottom line, because of the periodic continuation of the bonds, it is impossible to not have the inconsistent image flag warning.
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Thank you, now I understand it. But how could I solve this problem if I want to build a system with periodic continuation of the bonds.
Where is the problem with that? I don’t see one.