In all the books on qft that include a presentation of path integrals, the interacting Green function is expressed as a path integral with respect to the non-interacting (i.e., free) ground state of the system. Many books just simply write this formula without offering a proof, there are others that try hand-wavingly to justify this formula, and then there is the wonderful book "Quantum Many-particle Systems" by J.W. Negele and H. Orland, that on page 141 attempts to prove in minute details this formula, as their Eq. (3.8).
Free Hamiltonian:
H0|Φn⟩=Wn|Φn⟩
Full Hamiltonian:
H|Ψn⟩=(H0+V)|Ψn⟩=En|Ψn⟩
In Eq. (3.8) from page 141 of the book "Quantum Many-particle Systems" by J.W. Negele and H. Orland, there is an unfortunate error, namely
limT0→∞(−i)nζP|⟨Φ0|Ψ0⟩|2e−iE0T0∑l,m⟨Φ0|Ψl⟩⟨Ψl|e−i(12T0−tP(1))H˜aαP(1)e−i(tP(1)−tP(2))H˜aαP(2)×…˜aαP(2n)e−i(tP(2n)−12T0)H|Ψm⟩⟨Ψm|Φ0⟩
is equal to
limT0→∞(−i)nζP|⟨Φ0|Ψ0⟩|2e−iE0T0∑l,m⟨Φ0|Ψl⟩⟨Ψm|Φ0⟩×e−iT0(El−Em)2⟨Ψl|2n∏i=1˜a(H)αP(i)(tP(i))|Ψm⟩
and not to
limT0→∞(−i)nζP|⟨Φ0|Ψ0⟩|2e−iE0T0∑l,m⟨Φ0|Ψl⟩⟨Ψm|Φ0⟩×e−iT0(El+Em)2⟨Ψl|2n∏i=1˜a(H)αP(i)(tP(i))|Ψm⟩
One can see that El and Em enter the formula correctly as (El−Em) and not as (El+Em).
However, if one uses the correct formula, then one cannot obtain as a final result the interacting Green function
(−i)n⟨Ψ0|T[a(H)α1(t1)⋯a(H)αn(tn)a(H)†αn+1(tn+1)⋯a(H)†α2n(t2n)]|Ψ0⟩=Gn(α1t1,…,αntn|α2nt2n,…,αn+1tn+1)
How can one then obtain the interacting Green function for zero temperature as a path integral with respect to the free ground state?