The directory DDA should now contain a sample file
ddscat.par which
provides parameters to the program ddscat.
As provided (see AppendixA),
the file ddscat.par
is set up to calculate scattering by a 8
64
rectangular array of 192
dipoles, with an effective radius 
, at a wavelength of
(for a ``size parameter'' 
).
The dielectric function of the target material is provided in the file diel.tab, which is a sample file in which the refractive index is set to m=1.33+0.01i at all wavelengths; the name of this file is provided to ddscat by the parameter file ddscat.par.
The sample parameter file as supplied calls for the new GPFA FFT routine (GPFAFT) of Temperton (1992) to be employed and the PBCGST iterative method to be used for solving the system of linear equations. (See section §12 and §13 for discussion of choice of FFT algorithm and choice of equation-solving algorithm.)
The sample parameter file specifies (via option LATTDR) that the ``Lattice Dispersion Relation'' of Draine and Goodman (1993) be employed to determine the dipole polarizabilities. See §10 for discussion of other options.
The sample ddscat.par file specifies that the calculations be done for a
single wavelength () and a single effective radius
(
).
Note that in DDSCAT.5a the ``effective radius''
is the radius of a sphere of
equal volume - i.e., a sphere of volume 
, where d
is the lattice spacing
and N is the number of occupied (i.e., non-vacuum)
lattice sites in the target.
Thus the effective radius 
.
The incident radiation is always assumed to propagate along the x axis in
the ``Lab Frame''.
The sample ddscat.par file specifies incident polarization
state 
to be along the y axis
(and consequently polarization state 
will automatically be taken to be along the z axis).
IORTH=2 in ddscat.par
calls for calculations to be carried out for both incident polarization
states ( and
- see 20).
The target is assumed to have two vectors 
and 
embedded in it; is perpendicular to .
In the case of the 864 rectangular array of the
sample calculation, the vector is along the ``long'' axis of
the target, and the vector is along the ``intermediate'' axis.
The target orientation in the Lab Frame is set by three angles: 
,
, and 
, defined and discussed below in
§16.
Briefly, the polar angles
and specify the direction of in the Lab Frame.
The target is assumed to be rotated around by an angle .
The sample ddscat.par file
specifies 
and 
(see lines in ddscat.par
specifying variables BETA and PHI),
and calls for three values of the angle
(see line in ddscat.par specifying variable THETA).
DDSCAT.5a chooses values uniformly spaced in
; thus, asking for three values of between
0 and 
yields 
, 
, and .
Appendix A provides a detailed description of the
file ddscat.par.