  We present a method for calculation of Raman modes of the quantum solid phase
I solid hydrogen and deuterium. We use the mean-field assumption that the
quantised excitations are localized on one molecule. This is done by explicit
solution of the time-dependent Schroedinger equation in an angle-dependent
potential, and direct calculation of the polarisation. We show that in the
free-rotor limit, the H$_2$ and D$_2$ frequencies differ by a factor of 2,
which evolves toward $\sqrt{2}$ as the modes acquire librational character due
to stronger interactions. The ratio overshoots $\sqrt{2}$ if anharmonic terms
weaken the harmonic potential. We also use density functional theory and
molecular dynamics to calculate the E$_{2_g}$ optical phonon frequency and the
Raman linewidths. The molecular dynamics shows that the molecules are not free
rotors except at very low pressure and high temperature, and become like
oscillators as phase II is approached. We fit the interaction strengths to
experimental frequencies, but good agreement for intensities requires us to
also include strong preferred-orientation and stimulated Raman effects between
S$_0$(1) and S$_0$(0) contributions. The experimental Raman spectrum for phase
II cannot be reproduced, showing that the mean-field assumption is invalid in
that case.
