  In the calculation of the formation enthalpy of a defect, a stress term
accompanying the formation volume of the defect appears. This formation volume
is conventionally interpreted as the difference in volume between crystals with
and without the defect. When defect formation is investigated in the study of
crystal growth, the interpretation is sometimes misused by separately treating
the volumes of the defect and perfect crystal. However, these two quantities
are inseparable, and only the difference between them has physical reality.
