is widely used in the studies such as the Brueckner theory [@Brueckner1954Phys.Rev.95.217--228; @Day1967Rev.Mod.Phys.39.719--744; @Shen2019Prog.Part.Nucl.Phys.109.103713], we have developed the novel reconstituted SRG method [@Guo2019Phys.Rev.C99.054324], by replacing $M$ with $M^\ast$ in the non-relativistic expansion (with $M^\ast$ the Dirac mass of the particle). Consequently, the convergence of the reconstituted SRG method became much faster than that of the conventional one. It is worthwhile to mention that in parallel another work which started from the general operators and flow equation was carried out by Ren and Zhao [@Ren2019Phys.Rev.C100.044322], where the non-relativistic expansion was also carried out in the powers of ${1}/{M^\ast}$.

In addition to the improvement of the convergence, the single-particle densities $\rho_v(\bm{r})=\psi^\dag(\bm{r})\psi(\bm{r})$, which are also called the signle-particle vector or baryon densities, calculated by the novel reconstituted SRG are also more identical to the exact values than the conventional ones [@Guo2019Phys.Rev.C99.054324]. In contrast, in order to calculate the single-particle scalar densities $\rho_s(\bm{r})=\psi^\dag(\bm{r})\gamma_0\psi(\bm{r})$, the $\gamma_0$ matrix should be transformed in the same way as the Dirac Hamiltonian. However, in both the conventional and reconstituted SRG methods, the original Dirac Hamiltonian is operated by infinite steps of unitary transformation, and it is also worthwhile to point out that a crucial superiority of the SRG technique 