[Wien] cohere energy

[yulihua-wh at sohu.com]

Dear all users:
    I want to calculate the structure stability of two similar structures, one structure with one more atom than another structure. Using optimizing volume method, we can obtain the total-energy vs volume relation, but should not compare their equilibrium energy, due to they have different atoms number. I think that their cohere energy should be compared.
    My question is:
    How to calculate the cohere energy from total energy.

     Sincerely
     Yu lihua