True, but in many contexts, such as the above snippet, atom selection is not always "image aware" (shell code is), which may be an issue. Depending on the cell size, a 15 A shell based on lipid P and N atoms (assuming PC lipid) may be fairly reasonable.

For a loop over frames, the image cell boundary problem could also be handled by a coordinate translation in the xy plane followed by an image update, but that would slow things down a bit more.

I often try several different approaches in this situation (something to do with eggs and baskets smile ).

Rick Venable
computational chemist