We have performed ab initio pseudopotential calculations of three alternative structures of the GaAs(001) surface to examine the dependence of the reflectance anisotropy spectrum (RAS) upon the precise surface configuration. Spectra were calculated based upon a hypothetical (2 X 1) surface, a (2 X 4)-beta surface using experimentally measured atom positions, and a (2 X 4)-beta surface using atom positions given by total-energy minimization. It was found that the RA spectra depended significantly upon the atom positions, and in particular, that proper surface relaxation, including the Ga layer below the surface, was necessary in order to account for the low-energy (2.5 eV) feature in the experimentally observed spectrum.