  DDSCAT.5a is a freely available software package which applies the "discrete
dipole approximation" (DDA) to calculate scattering and absorption of
electromagnetic waves by targets with arbitrary geometries and complex
refractive index. The DDA approximates the target by an array of polarizable
points. DDSCAT.5a requires that these polarizable points be located on a cubic
lattice. DDSCAT.5a10 allows accurate calculations of electromagnetic scattering
from targets with "size parameters" 2 pi a/lambda < 15 provided the refractive
index m is not large compared to unity (|m-1| < 1).
  The DDSCAT package is written in Fortran and is highly portable. The program
supports calculations for a variety of target geometries (e.g., ellipsoids,
regular tetrahedra, rectangular solids, finite cylinders, hexagonal prisms,
etc.). Target materials may be both inhomogeneous and anisotropic. It is
straightforward for the user to import arbitrary target geometries into the
code, and relatively straightforward to add new target generation capability to
the package. DDSCAT automatically calculates total cross sections for
absorption and scattering and selected elements of the Mueller scattering
intensity matrix for specified orientation of the target relative to the
incident wave, and for specified scattering directions.
  This User Guide explains how to use DDSCAT.5a10 to carry out EM scattering
calculations. CPU and memory requirements are described.
