# Inconsistent data of chemical potential from different papers

I am looking for chemical potential from literature.$$^{1,2}$$ I compared the method and data they provided. They use the Fitted Elemental Reference Energy method via GGA+U on VASP employing PAW. The U values in these papers are a little different. As far as I understand from the papers, the chemical potential is equal to total energy of an isolated atom in a box (for example, 10x10x10). Am I right? Then I think it should be similar. However, I found that the data are somehow inconsistent. For example:

Element Paper 2 (Table 1) Paper 1 (Table V: Appendix)
Fe 2.200 -6.15
Mn 1.987 -7.00
Co 1.987 -4.75