Kawasaki and Onuki  proposed a disorder variable based on Steinhardt’s order paramaters which can be used to distinguish between ordered and disordered structures.
The disorder variable for an atom is defined as,
where S is given by,
l = 6 was used in the original publication as it is a good indicator of crystallinity. However, l = 4 can also be used for treating bcc structures. An averaged disorder parameter for each atom can also be calculated in pyscal,
In pyscal, disorder parameter can be calculated by the following code-block,
import pyscal.core as pc sys = pc.System() sys.read_inputfile('conf.dump') sys.find_neighbors(method='cutoff', cutoff=0) sys.calculate_q(6) sys.calculate_disorder(averaged=True, q=6)
|||Kawasaki .T, Onuki .A, JCP 135, 174109 (2011)|