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hi,
Do i need to allow "volume change" or "cell shape change" when i add impurities in a initially minimized bulk materials. (i want to compare the properties of few impurities in the bulk with each other, ofcourse with same parameters, and also study the surface of this bulk.)

ofcourse for surfaces volume change makes no sence but do i need to change the cell shape.


thanks for any comment.

in my opinion, that depends on how good your supercell approach characterizes the real material, especially concerning the concentration of the impurities (please mind that even a supercell with 1 impurity / 100 atoms corresponds to an impurity concentration of 1% !) and the structure of the material (please check the experimental data, if available): usually, volume and shape effects on the supercell become vanishingly small even for such mid-size systems, because significant relaxation is only found in the nearest and next-nearest neighbor shells surrounding the impurity (of course, depending on the kind of system you investigate). In any case, I would strongly recommend to make tests first: vary the supercell size and check the minimum size which is required to model the impurity system by the lattice of the host material.

thanks alot..