Pca2₁ ordering in a self-consistent model for solid orthohydrogen

A self-consistent method, described by Bell and Fairbairn in 1967, is used to investigate Pca2₁ ordering in solid orthohydrogen. It is shown that when the orientations of the orthohydrogen molecules are ordered in accordance with the Pca2₁ structure a state of very low energy is produced. The zero-order statistical approximation is applied to the transition between this ordered state and the high-temperature disordered phase. It is found that a second-order transition occurs at a temperature of 3·6°K.