We present very fast radiative transfer tools of interest for the interpretation of future spectroscopic observations taken with the GTC and its post-focus instrumentation. Our radiative transfer codes are based on the iterative methods introduced by Trujillo Bueno & Fabiani Bendicho (1995). Taking into consideration spherical geometry and macroscopic velocity fields, we present results from the application of these fast radiative transfer methods. We show why it is of interest to develop a medium-high resolution spectrograph in order to be able to resolve the rotational structure of the molecular bands. We show some spectropolarimetric observations made with the Tenerife Infrared Polarimeter (TIP), developed at the IAC and the theoretical modeling done with the theoretical tools we have developed.