  Motivated by recent experiments on isovalent-doped 1111 iron-based
superconductors LaFeAs$_{1-{x}}$P$_{x}$O$_{1-{y}}$F$_{y}$ and the theoretical
study that followed, we investigate, within the five orbital model, the
correlation between the spin fluctuation and the superconducting transition
temperature, which exhibits a double dome feature upon varying the Fe-As-Fe
bond angle. Around the first dome with higher $T_c$, the low energy spin
fluctuation and $T_c$ are not tightly correlated because the finite energy spin
fluctuation also contributes to superconductivity. On the other hand, the
strength of the low-energy spin fluctuation originating from the $d_{xz/yz}$
orbital is correlated with $T_c$ in the second dome with lower $T_c$. These
calculation results are consistent with recent NMR study, and hence strongly
suggest that the pairing in the iron-based superconductors is predominantly
caused by the multi-orbital spin fluctuation.
