So now it's time for me also to ask a question :) 1. The problem statement, all variables and given/known data I want to calculate a propagator for a coupled (degenerate) system, but somehow it's not possible to find normal coordinates. Maybe they don't exist, could be possible, it's been a time since I did that stuff. 2. Relevant equations C1, C2 are constant, the fields/coordinates are ψ and χ. The term in question is C1 ψ ∆ χ + C2 ψ², but it's under an integral sign and integration by parts is permissible. So one could also write - C1 (∇ψ) (∇χ) + C2 ψ², for example. 3. The attempt at a solution The cinetic term decouples in a pretty obvious way by using sum and difference coordinates, but this mixes the mass term. I was under the impression that one can always find normal coordinates, but maybe I'm wrong.