  An open source software package for modelling thermal neutron transport is
presented. The code facilitates Monte Carlo-based transport simulations and
focuses in the initial release on interactions in both mosaic single crystals
as well as polycrystalline materials and powders. Both coherent elastic (Bragg
diffraction) and incoherent or inelastic (phonon) scattering are modelled,
using basic parameters of the crystal unit cell as input.
  Included is a data library of validated crystal definitions, standalone tools
and interfaces for C++, C and Python programming languages. Interfaces for two
popular simulation packages, Geant4 and McStas, are provided, enabling highly
realistic simulations of typical components at neutron scattering instruments,
including beam filters, monochromators, analysers, samples and detectors. All
interfaces are presented in detail, along with the end-user configuration
procedure which is deliberately kept user-friendly and consistent across all
interfaces.
  An overview of the relevant neutron scattering theory is provided, and the
physics modelling capabilities of the software are discussed. Particular
attention is given here to the ability to load crystal structures and form
factors from various sources of input, and the results are benchmarked and
validated against experimental data and existing crystallographic software.
Good agreements are observed.
