Elementary question of binding energy

[td21]

Hello! I asked this question in Physics section but no one replies so i think it is more a chem question.

When it is given that the interacting energy between 2 ions in a crystal is U, is the binding energy W(for 1 atom) equal to U?
i.e. U=W?

I think it's a reasonable guess;but i also look in some books that "W=-(1/2)U"? Why?and which one is correct? I cannot find anything on the internet, thanks for your help.

My chemistry background is also weak so thanks for your help.

 

[Borek]

Interaction takes two ions, but you want energy per atom, yes?

Sign is just a matter of convention.