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DIFFUSION 991Carrying out the division in (A4) and retaining only the leading term givewhich is an excellent approximation, since the energy of formation of a vacancy is of the order of 1 ev, and therefore the higher terms in the series are very small. Qo is the partition function of a perfect crystal and Qi is the partition function of a crystal containing a vacancy.In the semiclassical approximation,where ypv^^vip^q) is the energy of a crystal containing * This paper represents a part of the thesis for the Master of Science degree in the field of Electrical Engineering by I.