The validity of ortho and para states of hydrogen at megabar pressures

It is well established that the single molecule ortho-para states of hydrogen remain valid in the low pressure solid. In this paper we show that on theoretical grounds the concept of ortho-para states for the molecules in a solid should remain valid to megabar pressures and probably until the Wigner-Huntington dissociative transition to the atomic solid is reached. We calculate the density dependence of the mixing of para states into ortho states and give a general discussion of the density dependence of conversion rates.