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Aiming at testing the validity of our magnesium atomic model andinvestigating the effects of non-local thermodynamical equilibrium (NLTE) onthe formation of the H-band neutral magnesium lines, we derive the differentialMg abundances from selected transitions for 13 stars either adopting orrelaxing the assumption of local thermodynamical equilibrium (LTE). Ouranalysis is based on high-resolution and high signal-to-noise ratio H-bandspectra from the Apache Point Observatory Galactic Evolution Experiment(APOGEE) and optical spectra from several instruments. The absolute differencesbetween the Mg abundances derived from the two wavelength bands are always lessthan 0.1 dex in the NLTE analysis, while they are slightly larger for the LTEcase. This suggests that our Mg atomic model is appropriate for investigatingthe NLTE formation of the H-band Mg lines. The NLTE corrections for the Mg IH-band lines are sensitive to the surface gravity, becoming larger for smallerlog g values, and strong lines are more susceptible to departures from LTE. Forcool giants, NLTE corrections tend to be negative, and for the strong line at15765 \AA\ they reach -0.14 dex in our sample, and up to -0.22 dex for otherAPOGEE stars. Our results suggest that it is important to include NLTEcorrections in determining Mg abundances from the H-band Mg I transitions,especially when strong lines are used.

NLTE Analysis of High Resolution H-band Spectra. II. Neutral Magnesium