Define here the atom position.
Translation
The coordinates of an atom can be set 
using the 
X, 
Y, 
Z entries.
Any decimal numbers will be accepted as good, even 
if they are outside the current range of visibility.
A simpler way to specify the atom position is to click on 
the screen, at the wished position, after defining all the other 
atom properties: a new atom object is created at that position.