The problem is in the line
(*neigh) = &(changed_atms[changed_atms.size() -1]);
You are taking the address of an element of a vector and keeping that
address long past the time it becomes invalid.
As soon as the vector grows, that address becomes invalid.
Even if the vector didn't grow, I think you are keeping the address
after the whole vector goes out of scope. But I'm not sure of that. I
am sure you are using the address while the vector is still in scope,
but after it may have grown. That's bad enough.
