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The positional derivatives of the two ion interactions in equation (115) has to be evaluated in order to calculate the exchange striction. Within the McPhase package the program makenn may perform this task and it will use analytical formulas. Here comes a typical code example:
REM create exchange interaction call makenn 7 -kaneyoshi 6 4.5 4 call copy results/makenn.j mcphas.j REM ... and its position derivatives using the -djd... option of makenn call makenn 7 -kaneyoshi 6 4.5 4 -djdx call makenn 7 -kaneyoshi 6 4.5 4 -djdy call makenn 7 -kaneyoshi 6 4.5 4 -djdz call copy results/makenn.djdx mcphas.djdx call copy results/makenn.djdy mcphas.djdy call copy results/makenn.djdz mcphas.djdz REM start mcphase with option -doeps to calculate strain call mcphasit -doeps