[cctbxbb] Structure factor calculation for Si

[Allan Lyckegaard]

Dear CCTBX-list

I am trying to reproduce the (scaled) intensities for Si reported here:
http://rruff.geo.arizona.edu/AMS/xtal_data/DIFfiles/15285.txt based on this
information: http://rruff.geo.arizona.edu/AMS/CIF_text_files/15285_cif.txt

I am using CCTBX in Python:

from cctbx.array_family import flex

from cctbx import sgtbx, crystal, miller, xray

sgnum = 227

sg = sgtbx.space_group_info(sgnum).group()

latticepar = [5.43070, 5.43070, 5.43070, 90, 90, 90]

dmin = 1.0

atompos = [['Si',(0.0, 0.0, 0.0)]]

scatterers = flex.xray_scatterer()

scatterers.append(xray.scatterer(label=atompos[0][0], site=atompos[0][1]))

pg = sg.build_derived_point_group()

sym = crystal.symmetry(space_group=sg,unit_cell=latticepar)

sps = crystal.special_position_settings(crystal_symmetry=sym)

structure =
xray.structure(special_position_settings=sps,scatterers=scatterers)

F = structure.structure_factors(d_min=dmin,algorithm='direct').f_calc()

list(F.amplitudes())


This gives:

[((0, 2, 2), 0.0),

 ((0, 4, 0), 120.21765812638594),

 ((1, 1, 1), 84.18849550806966),

 ((1, 3, 1), 65.48371727205928),

 ((1, 3, 3), 57.45438308324155),

 ((1, 5, 1), 51.48759686950681),

 ((2, 2, 2), 128.54540698861973),

 ((2, 4, 2), 0.0),

 ((3, 3, 3), 51.48759686950681)]

How come CCTBX give an amplitude of 0.0 for (0,2,2) and (2,4,2)? The
measured intensities in the DIFfile above does not show this. Am I missing
something?

Cheers,
/Allan
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