  The multidimensional topography of the collective potential energy function
of a so-called strong glass former (silica) is analyzed by means of classical
molecular dynamics calculations. Features qualitatively similar to those of
fragile glasses are recovered at high temperatures : in particular an intrinsic
characteristic temperature $T_c\simeq 3500$K is evidenced above which the
system starts to investigate non-harmonic potential energy basins. It is shown
that the anharmonicities are essentially characterized by a roughness appearing
in the potential energy valleys explored by the system for temperatures above
$T_c$.
