The difference between two methods of calculating the surface tension of alkali halide crystals, (i) from the surface stress and (ii) from the superficial derivative of the surface free energy, is examined. Emphasis is placed on the latter approach since Method (i) has recently been described in detail. The large discrepancies which previously existed between numerical values of the surface tension estimated from (i) and (ii) have been greatly reduced. In general the agreement between results obtained by the two approaches is quite close for undistorted crystals but the difference is increased when the effect of relaxation of the surface region is considered.
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