Reversed-frequency scalar contraction ($D_F(z{)}^{\ast }={\int }_p\frac{-i{e}^{-ipz}}{p^2-m^2-i0}$)

With $p\cdot x=Et-\mathbf{p}\cdot \mathbf{x}$, placing an annihilation operator on $e^{+ipx}$ rather than $e^{-ipx}$ makes its vacuum time-ordered contraction equal to $D_F^{\ast }$, with the opposite pole prescription. For positive time separation, the zero-spatial-momentum factor is $e^{+imt}/(2m)$ rather than $e^{-imt}/(2m)$. Replacing the integration variable $p$ by $-p$ does not change the sign of $i0$ and cannot fix the physical positive-energy boundary condition.