I want to calculate the D0 (Dissociation energy) of some diatomic molecules and the trivial procedure is calculating the energy of AB molecule and subtract it from the calculated-energy of A and B. If the AB has higher spin multiplicity than 1, it may make a question that what is the spin multiplicity of the dissociative products ? Are they all in their ground states as the free atoms or not ? the same question sometimes also may arise when the system has a closed shell. Certainly, the results of this trivial procedure depend on our selections and sometimes may cause wrong calculated-D0. Is there any standard method or tricky keyword in Firefly to overcome this problem or makes it easier to encounter ? I appreciate your help in advance.