Elastic constants and anisotropic pair correlations in solid hydrogen and deuterium'

'The ;~nisotropic displacement-displacemenlent correlation function for the two types o f pairs o f nearest neighhours in solid hcp hydrogen and deuterium is studied. Two mechanisms contributing to the deviation o f the pail-distribution function from axial symmetry around the pair asisare ~dentified. The one is clue to the anisotropy o f the phonon dispersion relations and is treated in 21 generalized Debye model parameterized in terms o f the elastic constants. The elasticity tensor is decomposetl into ~.otationnlly irreducible parts, ant1 certain new re l~~t ions between the elastic constants o f hcp crystals with central forces are derived. The other mechanism arises f~.om the irnniedinte, anisotropic environment o f a pair encl is treated using a generalized Einstein model. The relevance o f these results for the interpretation o f the microwave spectrum o f pairs o f or-thohyclrogen molecules in parahydrogen is ; r l w discussed.