  Photometric properties of various spectral libraries of synthetic stellar
spectra derived from the ATLAS 9 (Castelli et al. 1997), the PHOENIX NextGen
(Hauschildt et al. 1999a, 1999b), the NMARCS (Bessell et al. 1998), and the
BaSeL (Lejeune et al. 1998, Westera et al. 2002) models are discussed in view
of their application to the definition of the GAIA photometric system.
Comparisons with empirical UBVRI calibrations show significant discrepancies at
low temperatures between the different models, and with respect to the \basel
3.1 and the Alonso et al. (1998, 1999) empirical calibrations.
