Introduction

DDSCAT.5a is a Fortran software package to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries using the ``discrete dipole approximation'' (DDA). In this approximation the target is replaced by an array of point dipoles (or, more precisely, polarizable points); the electromagnetic scattering problem for an incident periodic wave interacting with this array of point dipoles is then solved essentially exactly. The DDA (sometimes referred to as the ``coupled dipole approximation'') was apparently first proposed by Purcell & Pennypacker (1973). DDA theory was reviewed and developed further by Draine (1988), Draine & Goodman (1993), and recently reviewed by Draine & Flatau (1994) and Draine (1999).

DDSCAT.5a is a Fortran implementation of the DDA developed by the authors. The previous version, DDSCAT.5a9, was released 1998 December 15. The current version, DDSCAT.5a10, released 2000 June 15, adds a new ``multisphere'' target option. DDSCAT.5a is intended to be a versatile tool, suitable for a wide variety of applications ranging from interstellar dust to atmospheric aerosols. As provided, DDSCAT.5a10 should be usable for many applications without modification, but the program is written in a modular form, so that modifications, if required, should be fairly straightforward.

The authors make this code openly available to others, in the hope that it will prove a useful tool. We ask only that:

• If you publish results obtained using DDSCAT, please consider acknowledging the source of the code.
• If you discover any errors in the code or documentation, please promptly communicate them to the authors.
• You comply with the ``copyleft" agreement (more formally, the GNU General Public License) of the Free Software Foundation: you may copy, distribute, and/or modify the software identified as coming under this agreement. If you distribute copies of this software, you must give the recipients all the rights which you have. See the file doc/copyleft distributed with the DDSCAT software.

We also strongly encourage you to send email to the authors identifying yourself as a user of DDSCAT; this will enable the authors to notify you of any bugs, corrections, or improvements in DDSCAT.

The current version, DDSCAT.5a10 , uses the DDA formulae from Draine (1988). The code incorporates Fast Fourier Transform (FFT) methods (Goodman, Draine, & Flatau 1991). The ``lattice dispersion relation'' (LDR) prescription (Draine & Goodman 1993) is used for determining dipole polarizabilities.

This User Guide assumes that you have already obtained the Fortran source code for DDSCAT.5a10 either via the World Wide Web (http://www.astro.princeton.edu/draine) or via anonymous ftp following the instructions in the README file. We refer you to the list of references at the end of this document for discussions of the theory and accuracy of the DDA [first see the recent reviews by Draine and Flatau (1994) and Draine (1999)]. In §4 we describe the principal changes between DDSCAT.5a10 and the previous releases. The succeeding sections contain instructions for:

• compiling and linking the code;
• running a sample calculation;
• understanding the output from the sample calculation;
• modifying the parameter file to do your desired calculations;
• specifying target orientation;
• changing the DIMENSIONing of the source code to accommodate your desired calculations.

The instructions for compiling, linking, and running will be appropriate for a UNIX system; slight changes will be necessary for non-UNIX sites, but they are quite minor and should present no difficulty.

Finally, this User Guide can be obtained by anonymous ftp from astro.princeton.edu, subdirectory draine/scat/ddscat/ver5a. For postscript, get either the uncompressed postscript file UserGuide.ps or the gzipped file UserGuide.ps.gz (remember to set the binary option in ftp before using the get command). Alternatively, you can get the LaTeX source UserGuide.tex.gz and the figures f1.eps.gz, f2.eps.gz, f3.eps.gz, f4.eps.gz, and f5.eps.gz.