# diffpy.srreal documentation¶

diffpy.srreal - calculators for PDF, bond valence sum and other pair quantities.

The diffpy.srreal package provides calculators for atomic pair distribution function (PDF), bond valence sums (BVS), atom overlaps for a hard-sphere model, bond distances and directions up to specified maximum distance. The atomic structure models are represented with internal classes as non-periodic, periodic or structures with space group symmetries. The package provides implicit adapters from diffpy.structure classes or from Crystal or Molecule objects from pyobjcryst. Adapters can be easily defined for any other structure representations in Python allowing their direct use with the calculators. Calculators support two evaluation models - BASIC, which performs a full pair-summation every time, and OPTIMIZED, which updates only pair contributions that have changed since the last evaluation. Calculations can be split among parallel jobs using Python multiprocessing package or any other library that provides parallel map function. PDF calculations can be done in two modes - either as a real-space summation of peak profiles (PDFCalculator) or as a reciprocal-space Debye summation and Fourier transform of the total scattering structure function (DebyePDFCalculator).

The diffpy.srreal package is a Python binding to the C++ library libdiffpy (https://github.com/diffpy/libdiffpy). Calculators are created as objects of a given calculator type and so multiple instances of the same calculator type can exist with different configurations. Calculators are composed of other objects that perform lower-level tasks, such as calculating peak profile or looking up atom scattering factors. These objects can be re-assigned at runtime allowing to easily customize the calculation procedure. New classes can be defined using object inheritance either in Python or in C++ and used with the existing calculators; as an example, this allows to calculate PDF with a user-defined profile function. A new calculator class can be also defined for any quantity that is obtained by iteration over atom pairs, by defining only the function that processes atom-pair contributions.

## Authors¶

diffpy.srreal is developed by members of the Billinge Group at Columbia University and at Brookhaven National Laboratory including Pavol Juhás, Christopher L. Farrow, Simon J.L. Billinge.

For a detailed list of contributors see https://github.com/diffpy/diffpy.srreal/graphs/contributors.