is the input for scalar product. ScalarProduct[p] is equivalent to ScalarProduct[p, p]. Expansion of sums of momenta in ScalarProduct is done with ExpandScalarProduct. Scalar product may be set, e.g., ScalarProduct[a, b] = ; but a and b must not contain sums. Note that ScalarProduct[a, b] = actually sets also: Pair[Momentum[a, ___], Momentum[b, ___]] = . It is encouraged to always set ScalarProduct's before any calculation. This improves the performance of FeynCalc .