# Why should vacuum energy be zero for LSZ formalism?

Can anyone explain why vacuum energy must be zero if we are to use LSZ formalism?

• Energy? Are you sure you don't mean $P^\mu \lvert \Omega \rangle = 0$ ? – John Rennie Dec 15 '18 at 12:08
• Isn't $P^u$ energy-momentum? – Mark Vanderbilt Dec 15 '18 at 12:36