4. METHOD OF SOLUTION
To evaluate the scattering kernel from the double differential scattering cross section, it is necessary to perform angular integration over scattering angle. This integra- tion is performed analytically in the THRUSH code. A "generalised" spectral density is used from which the coherent as well as the incoherent scattering effect can be calculated. The Sjoelander expansion or short time expansion is used to overcome the poor convergence at high incident neutron energy or high temperature.
The generally difficult to calculate inter-molecular elastic scattering from the atom pair of a different molecule is evaluated using Butler's model.
The present version of THRUSH accepts only isotropic "generalised" spectral density. Further improvements will be to accept delta- function and anisotropic spectral densities.