I am looking for Mg+Al metallic mixture binding energy with Dmol3 at which I need to be changing the aluminum and magnesium quantity in the allow

(I begen with optimizing a Mg metal HCP structure cell). With  CASTEP it is easy to do the work. But I need to do it with Dmol3  (meaning DFT theory) for several Mg and Al concentrations in the  mixtures. For example: I need 55%of Mg and 45% of Al; Mg0.55Al0.45. Let  us have another 90% of Mg and 10% of Al; Mg0.9Al0.1, and so on. When I  get the Dmol3 geometry optimization for both or any mixtures get exactly  the same correspondent energies in the outmol files, also I do not get  information about the Aluminum atoms, just talks say about Magnesium  atoms.

How do I make a Dmol3 geometry optimization of a compound  which chemical elements I am perceptual changing them, always ending  with a 100% total?

Best regards to all and thank you for your attention.