Pavel,
I've done this in the past using freud's GaussianDensity module in 2D, then taking the 2D FFT for the density to get one form of the structure factor. As Eric states, this is one of the many forms. This one is particularly useful for determining the crystal structure.
I don't know why freud's now built-in methods don't support 2D. You should check on the freud-users mailing list which may be monitored more closely by the authors of that code.
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Joshua A. Anderson, Ph.D.
Research Area Specialist, Chemical Engineering, University of Michigan
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