  The Magnus expansion is an efficient alternative to solving similarity
renormalization group (SRG) flow equations with high-order, memory-intensive
ordinary differential equation solvers. The numerical simplifications it offers
for operator evolution are particularly valuable for in-medium SRG
calculations, though challenges remain for difficult problems involving
intruder states. Here we test the Magnus approach in an analogous but more
accessible situation, which is the free-space SRG treatment of the spurious
bound-states arising from a leading-order chiral effective field theory (EFT)
potential with very high cutoffs. We show that the Magnus expansion passes
these tests and then use the investigations as a springboard to address various
aspects of operator evolution that have renewed relevance in the context of the
scale and scheme dependence of nuclear processes. These aspects include SRG
operator flow with band- versus block-diagonal generators, universality for
chiral EFT Hamiltonians and associated operators with different regularization
schemes, and the impact of factorization arising from scale separation.
Implications for short-range correlations physics and the possibilities for
reconciling high- and low-resolution treatments of nuclear structure and
reactions are discussed.
