The performance of plane wave basis sets to describe bond lengths, and Vibrational frequencies for diatomic anions is described within the context of ab initio total energy density functional pseudopotential method. Also, the behavior of the charge density as a function of the plane wave expansion size is studied for a molecule containing C, H, O, N, and S atoms. For the three properties studied, a critical size of the basis set that assures a reasonable description of them was found. These critical values are around a cutoff energy of at least 20 Ry below the cutoff energy used to design the pseudopotentials.