The accuracy requirements on atomic data for the calculation of stellar evolution with atomic diffusion are determined. In particular, the density of frequency grids needed for precise radiative acceleration (grad) calculations via the sampling method are presented. In order to minimize the number of frequency points needed for precise grad calculations, frequency grids that are more refined in the regions of the spectrum where the radiative flux is large are suggested. The following number of frequency points are needed for opacity table calculations to be used in stellar evolutionary codes including diffusion: 50,000 points for 4.20 ≤ log T ≤ 4.5, 30,000 points for 4.5 5.5. These opacity tables would render possible the study atomic diffusion in the exterior regions of certain chemically peculiar stars such as Ap or HgMn stars. In the sampling method, correction factors can be applied after the basic integrations over sampled spectra to include such effects as ion velocity averaging, redistribution of momentum among ions, and electron recoil during photoionization; these corrections are evaluated and illustrated for a few typical stellar models. Silicon is used as an example to show that the corrections are important mainly for T < 50,000 K. These corrections are used in stellar evolution calculations based on OPAL monochromatic opacity tables.