diff --git a/src/bath_correlation_functions/boson/Underdamped.jl b/src/bath_correlation_functions/boson/Underdamped.jl
index e55ce0dd..e68b58a3 100644
--- a/src/bath_correlation_functions/boson/Underdamped.jl
+++ b/src/bath_correlation_functions/boson/Underdamped.jl
@@ -34,5 +34,7 @@ function Boson_Underdamped_Matsubara(op, λ::Real, W::Real, ω0::Real, kT::Real,
         end
     end
 
-    return BosonBath(op, η_real, γ_real, η_imag, γ_imag)
+    δ = kT * W * λ^2 / ω0^4 - sum(η_real ./ γ_real)
+
+    return BosonBath(op, η_real, γ_real, η_imag, γ_imag, δ)
 end