I have read a piece of paper titled “ AuPdFe ternary solution model and applications to understanding the fO2 of hydrous, high-pressure experiments”. I am confused about how to calculate the activity of iron in the AuPdFe alloy. In the caption of Fig.3, the authors mentioned the △misfit is needed to incorporate into the solution model and the composition of the starting alloy is based on to calculate iron activity. Is the △misfit added directly to the right of equation 2? What to do with the composition of the starting alloy? Could anybody describe in detail how the iron activity 0.0039 is calculated taking the experiment B1129 in the appendix as an example? Thank you.