I was trying to work with the "calculate protein-protein ddG" tutorial. The tutorial comes with a resfile for mutating an Alanine with Tryptophan at residue 98. Following the steps gives, I get a ddG value of -6.6533 REU.
However, when I replace the line "98 D PIKAA W" with "98 D PIKAA A" (i.e. esentially replacing the alanine with alanine), I'm still getting a ddG value of 6.6643 REU. Can anyone point out why I might be getting a non-zero ddG even though I'm not even 'mutating' the proteins.