  Quantifying coronal magnetic field remains a central problem in solar
physics. Nowadays the coronal magnetic field is often modelled using nonlinear
force-free field (NLFFF) reconstructions, whose accuracy has not yet been
comprehensively assessed. Here we perform a detailed casting of the NLFFF
reconstruction tools, such as pi-disambiguation, photospheric field
preprocessing, and volume reconstruction methods using a 3D snapshot of the
publicly available full-fledged radiative MHD model. Specifically, from the MHD
model we know the magnetic field vector in the entire 3D domain, which enables
us to perform "voxel-by-voxel" comparison of the restored and the true magnetic
field in the 3D model volume. Our tests show that the available
pi-disambiguation methods often fail at the quiet sun areas dominated by
small-scale magnetic elements, while they work well at the AR photosphere and
(even better) chromosphere. The preprocessing of the photospheric magnetic
field, although does produce a more force-free boundary condition, also results
in some effective `elevation' of the magnetic field components. This
`elevation' height is different for the longitudinal and transverse components,
which results in a systematic error in absolute heights in the reconstructed
magnetic data cube. The extrapolations performed starting from actual AR
photospheric magnetogram are free from this systematic error, while have other
metrics comparable with those for extrapolations from the preprocessed
magnetograms. This finding favors the use of extrapolations from the original
photospheric magnetogram without preprocessing. Our tests further suggest that
extrapolations from a force-free chromospheric boundary produce measurably
better results, than those from the photospheric boundary.
