  Cool stars, such as M giants, can only be analysed in the near-infrared (NIR)
regime due to the ubiquitous TiO features in optical spectra of stars with Teff
< 4000 K. In dust obscured regions, like the inner bulge and Galactic Center,
the intrinsically bright M giants observed in the NIR is an optimal option to
determine their stellar abundances. Due to uncertainties in photometric
methods, a method to determine the stellar parameters for M giants from the NIR
spectra themselves is needed.
  We have carried out new observations of 44 M giant stars (also in APOGEE
DR17) with IGRINS (R=45,000) mounted on the Gemini South telescope. We also
obtained HK band IGRINS spectra of six nearby well-studied M giants from the
IGRINS spectral library. Using this sample, we have developed a method to
determine the stellar parameters for M giants from the NIR spectra by spectral
synthesis using SME. The method is validated using the six nearby well-studied
M-giants. We demonstrate the accuracy and precision by determining stellar
parameters and $\alpha$-element trends versus metallicity for solar
neighbourhood M giants.
  The effective temperatures that we derive (tested for 3400$\lesssim$ Teff
$\lesssim$4000\,K) agree excellently with the six nearby M giants which
indicates that the accuracy is indeed high. For the 43 solar neighborhood M
giants, our Teff, logg, [Fe/H], $\xi_\mathrm{micro}$, [C/Fe], [N/Fe], and
[O/Fe] are in unison with APOGEE with mean differences and scatter (our method
- APOGEE) of -67$\pm$33 K, -0.31$\pm$0.15 dex, 0.02$\pm$0.05 dex, 0.22$\pm$0.13
km/s, -0.05$\pm$0.06 dex, 0.06$\pm$0.06 dex, and 0.02$\pm$0.09 dex,
respectively. The $\alpha$-element trends versus metallicity for Mg, Si, Ca and
Ti are consistent with both APOGEE DR17 trends for the same stars as well as
with the GILD optical trends. We also find clear enhancement in abundances for
thick disc stars.
