  Quantum nuclear zero-point motions in solid H$_2$ and D$_2$ under pressure
are investigated at 80 K up to 160 GPa by first-principles path-integral
molecular dynamics calculations. Molecular orientations are well-defined in
phase II of D$_2$, while solid H$_2$ exhibits large and very asymmetric angular
quantum fluctuations in this phase, with possible rotation in the (bc) plane,
making it difficult to associate a well-identified single classical structure.
The mechanism for the transition to phase III is also described. Existing
structural data support this microscopic interpretation.
