Hello everyone. I have a question about free energies(delta G). Let's suppose, as an example, that we have following problem. We have PDB structure of complex of 2 proteins(lets say, Spike protein of SARS-CoV-2 and human ACE-2) and we mutate 1 aminoacid in Spike protein. Then we need to evaluate, how much does that mutation affect that complex(calculate delta delta G). As far as I understand, there are 2 different free energies: 1) energy of stability, 2) binding affinity. Roughly speaking, first one shows us how much energy we should spent to unfold a protein. The second one is about how much energy we should spent to unbind 2 molecules and take them apart.

My questions:

1. What free energy(the first or the second one) is it correct to calculate in my case(to evaluate how much virus transmissibility would change due to that mutation)?
2. Is it correct to calculate delta delta G of stability of a complex(2 proteins)? Or it's okay to do it only with one protein. As far as I know, FoldX can do it with command BuIldModel.
3. What is the physical meaning of a free energy of stability in a case of 2 proteins?
4. Do energy of stability and binding affinity somehow correlate?

I am newbie in structural bioinformatics, so maybe my questions would seen as stupid by someone. Thanks.