  We present a method of incorporating the discrete dipole approximation (DDA)
method with the point matching method to formulate the T-matrix for modeling
arbitrarily shaped micro-sized objects. The \emph{T}-matrix elements are
calculated using point matching between fields calculated using vector
spherical wave functions and DDA. When applied to microrotors, their discrete
rotational and mirror symmetries can be exploited to reduce memory usage and
calculation time by orders of magnitude; a number of optimization methods can
be employed based on the knowledge of the relationship between the azimuthal
mode and phase at each discrete rotational point, and mode redundancy from
Floquet's theorem. A 'reduced-mode' T-matrix can also be calculated if the
illumination conditions are known.
