Key points are not available for this paper at this time.
For pt.I see ibid., vol.8, p.2695 (1975). Bond energy curves for carbon and silicon are derived using a bonding orbital basis and a 'nearest-atom' approximation for the potential. When interactions between neighbouring atomic cells are included, good agreement is obtained with observed bond lengths, bulk moduli and bond energies.
D.W. Bullett (Sun,) studied this question.