  We introduce a thermodynamical model based on the two-temperature approach in
order to fully understand the dynamics of the coherent A$_{1g}$ phonon in
laser-excited bismuth. Using this model, we simulate the time evolution of
(111) Bragg peak intensities measured by Fritz {\it{et al}} [Science {\bf 315},
633 (2007)] in femtosecond X-ray diffraction experiments performed on a bismuth
film for different laser fluences. The agreement between theoretical and
experimental results is striking not only because we use fluences very close to
the experimental ones but also because most of the model parameters are
obtained from {\it{ab-initio}} calculations performed for different electron
temperatures.
