  Ultraviolet over-ionisation of iron atoms in the solar atmosphere leads to
deviations in their level populations from the Saha-Boltzmann statistics. This
causes their line profiles to form in Non-Local Thermodynamic Equilibrium
(NLTE) conditions. While inverting such profiles to determine atmospheric
parameters, the NLTE effects are often neglected and deviations from LTE are
compensated for by tweaking other quantities. We investigate how the routinely
employed LTE inversion of iron lines formed in NLTE under- or over-estimates
atmospheric quantities such as temperature (T), line-of-sight velocity (v_LOS),
magnetic field strength (B) and inclination (gamma) while the previous papers
have focused mainly on T. We synthesize the Stokes profiles of Fe I 6301.5 A
and 6302.5 A lines in both LTE and NLTE using a snapshot of a 3D MHD
simulation. The profiles are then inverted in LTE. By considering the
atmosphere inferred from inversion of LTE profiles to be the fiducial model, we
compare atmosphere from the inversion of NLTE profiles with it. Any differences
observed are attributed to NLTE effects. Neglecting the NLTE effects introduces
errors in the inverted atmosphere. While the errors in T can go up to 13%, in
v_LOS and B the errors can be as high as 50% or more. We find these errors to
be present at all three inversion nodes. Importantly, they survive degradation
from spatial averaging of the profiles. We give an overview of how the neglect
of NLTE effects influences the values of T, v_LOS, B and gamma determined by
inverting Fe I 6300 A line pair, as observed, e.g., by Hinode. Errors are found
at the sites of granules, intergranular lanes, magnetic elements, basically in
every region susceptible to NLTE effects. For an accurate determination of
atmospheric quantities and their stratification, it is therefore important to
take account of NLTE effects.
