  Methanol-water liquid mixtures have been investigated by high-energy
synchrotron X-ray and neutron diffraction at low temperatures. We are thus able
to report the first complete sets of both X-ray and neutron weighted total
scattering structure factors over the entire composition range (at 12 different
methanol concentrations (x$_M$) from 10 to 100 mol%) and at temperatures from
ambient down to the freezing points of the mixtures. The new diffraction data
may later be used as reference in future theoretical and simulation studies.
The measured data are interpreted by molecular dynamics simulations, in which
the all atom OPLS/AA force field model for methanol is combined with both the
SPC/E and TIP4P/2005 water potentials. Although the TIP4P/2005 water model was
found to be somewhat more successful, both combinations provide at least
semi-quantitative agreement with measured diffraction data. From the simulated
particle configurations, partial radial distribution functions, as well as
various distributions of the number of hydrogen bonds have been determined. As
a general trend, the average number of hydrogen bonds increases upon cooling.
However, the number of hydrogen bonds between methanol molecules slightly
decreases with lowering temperatures in the concentration range between ca. 30
and 60 mol % alcohol content. The same is valid for water-water hydrogen bonds
above 70 mol % of methanol content, from room temperature down to 193 K.
