Crystal field and related phenomena in hexagonal close-packed H2 and D2   under pressure
  The lattice distortion parameter (the deviation of the c/a ratio from the ideal value 1.633), orientational order parameter, and crystal-field parameter in hexagonal close-packed (hcp) lattice of p-H2, o-D2 and n-H2 are calculated using the semi-empirical lattice-dynamic approach. It is shown that the lattice distortion in the J-even species is two order of magnitude smaller compared with that found in n-H2, and n-D2. The difference is due to the splitting of the J-odd rotational levels in the J-even - J-odd mixtures. 
Yu. A. Freiman and S. M. Tretyak and Alexei Grechnev and Alexander F.   Goncharov and Russell J. Hemley
cond-mat.mtrl-sci