Ground State Energy Difference of Hcp and Fcc Ortho-Hydrogens

The lowest state of the quadrupole-quadrupole interaction is studied for hcp and fcc ortho-hydrogens. The difference of the classical ground state energies of hcp and fcc ortho-hydrogens is estimated to be 0.72 °K per molecule by use of the previous result for the fcc structure given by one of us. The zero-point energy corrections are studied also for both hcp and fcc structures by a perturbation calculation. The resulting ground state energy difference is estimated to be 0.57 °K per molecule.