For example consider $2\to2$ scattering in scalar $\phi^4$ theory. When the in/out coming positions are fixed, it is easy to calculate in terms of contractions. When we fix the incoming / outgoing momenta instead, how can we use the Feynman rules in that case?
In principle one can expand all the fields in terms of creation/annihilation operators and pull them through but how is this procedure generalised using the knowledge on position space correlation functions?
