Discrete-dipole approximation with polarizabilities that account for both finite wavelength and target geometry

The discrete-dipole approximation (DDA) is a powerful method for calculating absorption and scattering by targets that have sizes smaller than or comparable to the wavelength of the incident radiation. We present a new prescription -- the Surface-Corrected Lattice Dispersion Relation (SCLDR) -- for assigning the dipole polarizabilities that takes into account both target geometry and finite wavelength. We test the SCLDR in DDA calculations using spherical and ellipsoidal targets and show that for a fixed number of dipoles, the SCLDR prescription results in increased accuracy in the calculated cross sections for absorption and scattering. We discuss extension of the SCLDR prescription to irregular targets.