  Large grids of synthetic spectra covering a widespread range of stellar
parameters are mandatory for different stellar and (extra-)Galactic physics
applications. Such large grids can be used for the automatic parametrisation of
stellar spectra such as that performed within the AMBRE project for which the
main goal is the stellar atmospheric parameters determination for the few
hundreds of thousands of archived spectra of four ESO spectrographs. To fulfil
the needs of AMBRE and future similar projects, we have computed a grid of
synthetic spectra over the whole optical domain for cool to very cool stars of
any luminosity with metallicities varying from 10-5 to 10 times the solar
metallicity, and considering large variations in the chemical content of the
{\alpha}-elements. New generation MARCS model atmospheres and the Turbospectrum
code for radiative transfer have been used. We have also taken into account as
complete as possible atomic and molecular linelists. A new grid of 16783 high
resolution spectra over the wavelength range 3000 to 12000 {\AA} has been
computed with a spectral resolution always larger than 150000. Normalised and
absolute flux versions are available over a wide range of stellar atmospheric
parameters for stars of FGKM spectral types. The covered parameters are
2500K\leqTeff\leq8000K, -0.5\leqlog(g)\leq5.5dex, -5.0\leq[M/H]\leq+1.0 dex and
five different values of the enrichment in {\alpha}-elements have been
considered (0.0, \pm0.2 dex and \pm0.4 dex around the standard values). This
grid is made publicly available through the POLLUX database (about 50% of the
spectra are already included in this database) and in FITS format upon request
to the authors.
