DDSCAT contains routines to generate dipole arrays representing targets of various geometries, including spheres, ellipsoids, rectangular solids, cylinders, hexagonal prisms, tetrahedra, two touching ellipsoids, and three touching ellipsoids. The target type is specified by variable CSHAPE on line 7 of ddscat.par, and up to 6 parameters (SHPAR1, SHPAR2, SHPAR3, ...) on line 8. The target geometry is most conveniently described in a coordinate system attached to the target which we refer to as the ``Target Frame'' (TF), so in this section only we will let x,y,z be coordinates in the Target Frame. Once the target is generated, the orientation of the target in the Lab Frame is accomplished as described in §16.
Target geometries currently supported include:
,
where d is the interdipole spacing.
A sphere is obtained by setting
SHPAR1 = SHPAR2 = SHPAR3 = diameter/d.
The target axes are set to 
and 
in
the TF.
The user must set NCOMP=1 on line 9 of ddscat.par.
and in the TF.
User must set NCOMP=1.
and in the TF.
User must set NCOMP=1.
in the TF, and prism
axis in the TF
normal to one of the rectangular sides
of the hexagonal prism.
User must set NCOMP=1.
[emerging from one vertex] and
[emerging from an edge] in the TF.
User must set NCOMP=1.
[along line connecting ellipsoids]
and .
User must set NCOMP=2 and provide dielectric function file names
for both ellipsoids.
Ellipsoids are in order of increasing x.
and
in the TF.
User must set NCOMP=6 and provide x, y, z dielectric functions for
first ellipsoid, and x, y, z dielectric functions for second
ellipsoid (ellipsoids are in order of increasing x).
and in the TF.
User must set NCOMP=3 and provide xx, yy, zz elements of
the (diagonal)dielectric tensor.
parallel to
cylinder axis, diel.func. 2 for perp. to axis.
Cylinder axis =, .
User must set
NCOMP=2 and provide parallel, perpendicular dielectric functions.
along line of ellipsoid centers,
.
User must set NCOMP=3 and provide (isotropic) dielectric
functions for first, second, and third ellipsoid.
along line of ellipsoid centers,
.
User must set NCOMP=9 and provide x, y, z dielectric function
for first ellipsoid, x, y, z diel. func. for second ellipsoid,
and x, y, z diel. func. for third ellipsoid
(ellipsoids are in order of increasing x).
, in TF.
User must set NCOMP=2 and provide delectric functions for
``core" and ``mantle".
,
where 
and 
are basis vectors in TF, and SHPAR5
is in degrees.
If SHPAR6=0., then target axes ,
.
If SHPAR6=1., then axes 
and 
are set to
principal axes with largest and 2nd largest moments of inertia assuming
spheroids to be of uniform density.
User must set NCOMP=2 and provide dielectric function files for
each spheroid.
,
.
If PRIN=1, then and are set
to principal
axes with largest and second largest moments of inertia,
assuming target to be of uniform density.
User must set NCOMP=1.
and are principal axes with largest and
second-largest moments of inertia.
User must set NCOMP=1.
Target size is controlled by shape parameter PAR1 =
width of one block in lattice units.
, 
in Target Frame.
(largest I) and 
(intermediate I).
(arb. units)
(arb. units)
(arb. units)
, 
, 
are the coordinates of the center,
and 
is the radius of sphere j., , , (j=1,...,N) establish only
the shape of the N-sphere target.
The actual size (in units of 
) is set by the value
of 
specified in ddscat.par, where as
always 
, where V is the
volume of material in the target.The user should be able to easily modify these routines, or write new routines, to generate targets with other geometries. The user should first examine the routine target.f and modify it to call any new target generation routines desired. Alternatively, targets may be generated separately, and the target description (locations of dipoles and ``composition" corresponding to x,y,z dielectric properties at each dipole site) read in from a file by invoking the option FRMFIL in ddscat.f.
It is often desirable to be able to run the target generation routines
without running the entire DDSCAT code.
We have therefore provided a program CALLTARGET
which allows the user to generate targets interactively; to create this
executable just type
make calltarget . The program calltarget is to be run interactively; the prompts are self-explanatory. You may need to edit the code to change the device number IDVOUT as for DDSCAT (see §5.1 above).
After running, calltarget will leave behind an ASCII file target.out which is a list of the occupied lattice sites in the last target generated.