Context : We present work on H \alpha spectral line characteristics in PHOENIX stellar model atmospheres and their comparison to microlensing observations .

Aims : We examine in detail the H \alpha equivalent width ( EW ) and the line shape characteristics for effective temperatures of 4500 K < T _ { eff } < 5600 K where H \alpha is a strong spectral feature .

Methods : We find that H \alpha EW in models calculated under the assumption of local thermodynamic equilibrium ( LTE ) is up to 15 % smaller than in models without this assumption , non-LTE models ( NLTE ) and that line shapes vary significantly for the two model types .
A comparison with available high quality microlensing data , capable of tracing H \alpha absorption across the face of one G5III giant , shows that the LTE model that fits the EW best is about 100K hotter than and the best-fitting NLTE model has a similar T _ { eff } as predicted by the spectral type analysis of the observed star but agree within the uncertainties of the observationally derived temperature .

Results : Neither LTE nor NLTE models fit the line shape well .
We suspect unmodelled chromospheric emission .
Line shape diagnostics suggest lower gravities than derived for the star and are unacceptable low in the case of the LTE models .
We show that EW alone is insufficient for comparison to stellar model atmospheres , but combined with a new shape parameter we define is promising .
In stellar parameter ranges where the H \alpha line is strong , a NLTE approach of modeling stellar atmospheres is not only beneficial but mandatory .

Conclusions :