text: The shape of a powder diffraction reflection is influenced by the characteristics of the beam, the experimental
  arrangement, and the sample size and shape. In the case of monochromatic neutron sources the convolution
  of the various effects has been found to result in a reflex almost exactly Gaussian in shape. If this
  distribution is assumed then the contribution of a given reflection to the profile  at position 
  is:
  where  is the full width at half peak height (full-width half-maximum),  is the center of the reflex, and  is the calculated intensity of the reflex (determined from the structure factor,
  the Lorentz factor, and multiplicity of the reflection).
  At very low diffraction angles the reflections may acquire an asymmetry due to the vertical divergence of the beam.
  Rietveld used a semi-empirical correction factor,  to account for this asymmetry:
  where  is the asymmetry factor and  is , , or  depending on the difference  being positive, zero, or negative respectively.
  At a given position more than one diffraction peak may contribute to the profile. The intensity is simply the sum of all reflections contributing at the point .