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Asymmetric Unit


The term "asymmetric unit" refers to the minimum number of atoms necessary to uniquely define a crystal structure.

To define a crystal, one could list all the atoms inside one unit cell. However, if the crystal has some symmetry then some of these atoms will be related to other atoms by the operation of symmetry elements.

So, a more concise way of defining a crystal structure is to specify the crystal symmetry (i.e., its spacegroup), and then give the smallest subset of the unit cell (i.e., the asymmetric unit) which, when combined with the symmetry "generates" the remainder of the unit cell.


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