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Manuscript Title: II. A computer program for calculation of parameters necessary for the computation of reliable pair distribution functions of non- crystalline materials from limited diffraction data.
Authors: F.Y. Hansen
Program title: FYPAR
Catalogue identifier: ACYS_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 15(1978)417
Programming language: Fortran.
Computer: IBM 370/168.
Operating system: OS/VS2, 4MVS.
RAM: 15K words
Word size: 32
Peripherals: disc.
Keywords: Solid state physics, Non-crystalline, Pair distribution Function, Structure factor, Fourier transform, Truncation error, Scattering, Neutron, Experiment.
Classification: 7.4, 7.6.

Subprograms used:
Cat Id Title Reference
ACYR_v1_0 FYINT CPC 15(1978)401
ACYT_v1_0 PAR CPC 15(1978)431

Nature of problem:
The pair distribution function of non-crystalline materials may be obtained by a Fourier transformation of the structure factor as calculated inpart I of this series. The structure factor is often limited in the sense that it shows significant oscillations at the maximal wave vector transfer obtainable. The Fourier transformation of such functions, therefore introduces truncation errors in the transformed function. With this program a parametrization of the small distance part of the pair distribution function is obtained according to a method described by us, and the final pair distribution function without truncation errors may then be calculated with the program described in part III of this series using the parameters calculated with this program.

Restrictions:
The only restriction of the program is the high speed storage available.

Unusual features:
The storage requirement depends on the number of structure factor data and the number of peaks used to resolve the small distance part of the pair distribution function. In the present set-up the storage requirement is set to 15083 words, which is estimated to be satisfactory for a large number of cases.

Running time:
The running time depends on the number of structure factor data and the number of peaks in the least squares fit calculation. For the test example the running time is 8.2 s. The compilation time is included in this time, so if a compiled version of the program is used, the running time is reduced to 4.1 s.