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List of differences between native QE and SIRIUS implementations:

• SIRIUS is using spline interpolation and integration everywhere; QE is using Simpson's integration to evaluate radial integrals and some sort of cubic interpolation for radial integrals (see, for example, lines 138-150 in qvan2.f90)
• for real functions such as density and magnetization SIRIUS is always storing half of the plane-wave coefficients (due to the symmetry rho(G) = rho^{*}(-G)); QE does this trick only for Gamma-point calculation
• SIRIUS generates density from the valence wave-functions on a coarse FFT mesh; density is then transformed to G-domain, augmented in G-domain, and then transformed back to the fine-grained FFT grid; QE always generates density on the fine-grained FFT grid
• for the stress tensor: QE computes derivatives of spherical harmonics over the cartesian components of G-vectors numerically; SIRIUS uses analytical expression