where k and k z control the strength of the confinement in the x, y and z directions respectively. In addition, a linear term in ẑ is added with strength C z = -0.015167 Hartree Bohr -1 that serves to localize the excess electron on one side of the molecule. The +z direction was always chosen to point from the oxygen atom to the electron. The high order of the position operators in Eq. 1 introduces a steep repulsive wall surrounding the water molecule that confines the excess electron. This is needed since a single water molecule alone does not bind an excess electron. A value of k = 1 × 10 -7 Hartree Bohr -8 confines the excess electron within a few Å of the water, while leaving the neutral water's electronic structure essentially unperturbed. k z is then varied between 1 × 10 -7 and 1 × 10 -10 Hartree Bohr -8 , which has the effect of moving the minimum in the confining potential, and therefore the excess electron's density, to increasing displacement from the water molecule (see dashed red curves in Fig. ).