I was planning to implement the GGA+U correction scheme to study some compounds containing transition metals. To choose the U-values, I read the following page:
However, I was confused about one thing. The page states that they have used the method outlined by Wang et al.'s work, and the reactions they analyzed in order to find the U-values are identical to that of which Wang et al. used. However, when looking at iron, the U-value listed on the page is 5.3 eV, as opposed to about 4.0 eV obtained in the work of Wang et al. This seems like a surprisingly large difference, given that the same methods were used. A few other elements have also have slightly different results, but are in much better agreement.
Is there a reason for this large discrepancy of the Fe U-values?
Any assistance or clarification would be greatly appreciated.