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Manuscript Title: III. A computer program for calculation of reliable pair distribution functions of non-crystalline materials from limited diffraction data.
Authors: F.Y. Hansen
Program title: PAR
Catalogue identifier: ACYT_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 15(1978)431
Programming language: Fortran.
Computer: IBM 370/168.
Operating system: OS/VS2, 4MVS.
RAM: 9K words
Word size: 32
Peripherals: disc.
Keywords: Solid state physics, Non-crystalline, Pair distribution Function, Structure factor, Truncation error, Scattering, Neutron, Experiment.
Classification: 7.4, 7.6.

Subprograms used:
Cat Id Title Reference
ACYR_v1_0 FYINT CPC 15(1978)401
ACYS_v1_0 FYPAR CPC 15(1978)417

Nature of problem:
This program calculates the final pair distribution function of non- crystalline materials on the basis of the experimental structure factor as calculated in part I of this series and the parameters of the small distance part of the pair distribution function as calculated in part II of this series. In this way, truncation errors may be eliminated from the final pair distribution function.

Solution method:
The calculations with this program depend on the results calculations with the programs described in parts I and II in this series (Comp. Phys. Commun. 15(1978)401, 15(1978)417). The final pair distribution function is calculated by a Fourier transform of a combination of an experimental structure factor and a model structure factor.

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

Unusual features:
The storage requirement depends on the number of data points in the structure factor, the number of data points in the final pair distribution function and the number of peaks necessary to resolve the small distance part of the pair distribution function. In the present set-up a storage requirement is set to 8860 words which is estimated to be satisfactory for a large number of cases.

Running time:
The running time depends on the number of data points in the structure factor and the number of data points in the final pair distribution function. For the test example, the running time is 1.4 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 0.75 s.