  In this manuscript we investigate the capabilities of the Discrete Dipole
Approximation (DDA) to simulate scattering from particles that are much larger
than the wavelength of the incident light, and describe an optimized publicly
available DDA computer program that processes the large number of dipoles
required for such simulations. Numerical simulations of light scattering by
spheres with size parameters x up to 160 and 40 for refractive index m=1.05 and
2 respectively are presented and compared with exact results of the Mie theory.
Errors of both integral and angle-resolved scattering quantities generally
increase with m and show no systematic dependence on x. Computational times
increase steeply with both x and m, reaching values of more than 2 weeks on a
cluster of 64 processors. The main distinctive feature of the computer program
is the ability to parallelize a single DDA simulation over a cluster of
computers, which allows it to simulate light scattering by very large
particles, like the ones that are considered in this manuscript. Current
limitations and possible ways for improvement are discussed.
