With the confining potential of Eq. 1, electronic structure calculations were then performed on a water anion. Since the excess electron localizes up to several Ångstom from the water, it is important to augment the standard atom-centered basis functions to describe highly diffuse orbitals. Following our previous work, we achieved this by adding to the aug-cc-pVTZ basis 38 a cubic grid of floating s type primitive Gaussian functions with exponent 0.16 Bohr -2 . The grid contains 7 × 7 × 13 functions with a spacing of 2.5 Bohr. The electron-water interaction energy is defined as: