In our plane-wave calculations, The valence electron interactions with the atomic H, C, N, O, P, S, and Cl cores were approximated using generalized norm-conserving Hamann pseudopotentials modified into a separable form as suggested by Kleinman and Bylander. Restricted calculations with aperiodic free space boundary condition method of Bylaska et al. were performed with a wavefunction cutoff energy of 100 Ry and a density cutoff energy of 200 Ry.