Before settling on the final choice of *f* as described in
Eq. () three separate forms were experimented with.

- A spherical
*f*function that was cut off by a Fermi function at the edge of the largest sphere that could be inscribed within the Wigner-Seitz cell, i.e.where is the radius of the above sphere. This function was thought at the time to be more elegant as it removed any discontinuity in the gradient of

*f*as an electron moves out of one side of the simulation cell and back in the opposite side. - A spherical
*f*function with a sharp cut off at the edge of the largest sphere that could be inscribed within the Wigner-Seitz cell, i.e. - The
*f*function described by Eq. (), which is not spherically symmetrical, and has a sharp cut off at the edge of the Wigner-Seitz cell.

The discontinuity in the derivative of the interaction introduced by
this choice of *f* only introduces a discontinuity into the third
derivative of the exact wavefunction. This does not contribute to the
kinetic energy of the system and is therefore harmless.

Tue Nov 19 17:11:34 GMT 1996