Left: comparison of log(gf) between Babushkin and Coulomb gauges for ab initio (black plus) and fine-tuned (red plus) results, respectively. Insets (a1) and (a2) show the histograms of the log(gf) differences for transitions with log(gf) ≥ −2 and log(gf) < −2, respectively. Right: comparison of log(gf) between ab initio and fine-tuned calculations for Babushkin (black diamond) and Coulomb gauges (red hexagon), respectively. Insets (b1) and (b2) show the histograms of the log(gf) differences for transitions with log(gf) ≥ −3 and log(gf) < −3, respectively.