Van der Waals' Forces and the Vapor Pressures of Ortho- and Parahydrogen and Ortho- and Paradeuterium

The nonexchange interaction energies of pairs of orthohydrogen and of parahydrogen molecules are examined in detail. The first-order contribution is seen to contribute the specific heat anomaly of o-hydrogen at low temperatures. The second-order interaction is computed for cases where the first-order contribution is zero. Its contribution, calculated with explicit recognition of the influence of rotational energy on the electronic states, is shown to be of the correct sign and order of magnitude to account for the vapor pressure and volume differences of o- and p-hydrogen and deuterium. More exact confirmation cannot be obtained because of the approximations and the disturbing influence of the anharmonic zero-point vibrations.