“…In particular, a formula for calculating the energy of disappearance of the y ($) curve minima has not been derived. Therefore, Onderdelinden's (1968) formula obtained for sputtering on the basis of the channelling concepts was used in some work (Sommerfeldt et a1 1971b, Brusilovsky andMolchanov 1971) to calculate Ec. Onderdelinden's formula, however, was derived disregarding the close-packed planes and, therefore, gives a value of Ec which sometimes disagrees with experiment.…”