In addition to solving the scattering problem for a dipole array, DDSCAT.5a\
can compute the three-dimensional force 
and torque 
exerted on this array by
the incident and scattered radiation fields. This calculation is carried out,
after solving the scattering problem, provided DOTORQ has been
specified in ddscat.par.
For each incident polarization mode,
the results are given in terms of dimensionless efficiency vectors
and 
, defined by
where and are the time-averaged
force and torque on the dipole array,
is the wavenumber in vacuo,
and 
is the time-averaged energy density for
an incident plane wave with amplitude
.
The radiation pressure efficiency vector can be written
where 
is the direction of propagation of the incident radiation,
and the vector g is the mean direction of propagation of the
scattered radiation:
where 
is the element of solid angle in scattering direction 
,
and 
is the differential scattering cross section.
Equations for the evaluation of the radiative force and torque
are derived by Draine & Weingartner (1996).
It is important to note that evaluation of and
involves averaging over scattering directions to
evaluate the linear and angular momentum transport by the scattered
wave.
This evaluation requires appropriate choices of the parameters
ICTHM and IPHM - see §21.