I think you are mixing up things here. To calculate the CDD, you need:
1) the Al/Fe cell;
2) the Al fragment (using the same cell as 1);
3) the Fe fragment (using the same cell as 1).
Therefore, if you are able to calculate the Al/Fe system, you should also be able to calculate the fragments to extract the CDD. The required cutoff for 2 and 3 will be that used for 1.
Daniele.