  DDSCAT 7.3 is an open-source Fortran-90 software package applying the
discrete dipole approximation to calculate scattering and absorption of
electromagnetic waves by targets with arbitrary geometries and complex
refractive index. The targets may be isolated entities (e.g., dust particles),
but may also be 1-d or 2-d periodic arrays of "target unit cells", allowing
calculation of absorption, scattering, and electric fields around arrays of
nanostructures.
  The theory of the DDA and its implementation in DDSCAT is presented in Draine
(1988) and Draine & Flatau (1994), and its extension to periodic structures in
Draine & Flatau (2008), and efficient near-field calculations in Flatau &
Draine (2012). DDSCAT 7.3 includes support for MPI, OpenMP, and the Intel Math
Kernel Library (MKL). DDSCAT supports calculations for a variety of target
geometries. Target materials may be both inhomogeneous and anisotropic. It is
straightforward for the user to "import" arbitrary target geometries into the
code. DDSCAT automatically calculates total cross sections for absorption and
scattering and selected elements of the Mueller scattering intensity matrix for
user-specified scattering directions. DDSCAT 7.3 can efficiently calculate E
and B throughout a user-specified volume containing the target.
  This User Guide explains how to use DDSCAT 7.3 to carry out electromagnetic
scattering calculations, including use of DDPOSTPROCESS, a Fortran-90 code to
perform calculations with E and B at user-selected locations near the target. A
number of changes have been made since the last release, DDSCAT 7.2 .
