  By following the conventional similarity renormalization group (SRG)
expansion of the Dirac equation developed in [J.-Y. Guo, Phys. Rev. C
\textbf{85}, 021302 (2012)], we work out the analytic expression of the
${1}/{M^4}$ order and verify the convergence of this method. As a step further,
the reconstituted SRG method is proposed by using the re-summation technique.
The speed of convergence of the reconstituted SRG becomes much faster than the
conventional one, and the single-particle densities with the reconstituted SRG
are also almost identical to the exact values.
