thank you for yours suggestions.
I implemented 8-dimensional operator with 4 fermions and 2 derivatives (via an auxiliary field). Everything seems work well expect when I have to convert the amplitude.
This time, there are some momentum on the numerator and I think that the FCFAConvert function doesn't convert properly the scalar products. In fact, I get some terms like (-k1).(k1 - p2) which is different form -Pair[Momentum[k1], Momentum[k1 - p2]]. With this last expression, I can simplify my expression using ExpandScalarProduct and using Mandelstam variables.
As usual, you find my model and the notebook here https://www.dropbox.com/sh/hm77oyedcb3p055/AABzBlEPZvcPjAstjoclCR3ya?dl=0
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