One possible explanation for the new function (
) in the Jastrow
factor being unable to reduce the energy and variance of energy
significantly is that there are too many high powers in the prefactor
of the Chebyshev expansion and these reduce its ability to produce a
good fit.
As the electron density close to the ion cores is very low compared
to that in the bond (see Figure ), it is possible that
removing the 
prefactor to the Chebyshev
expansion will not cause any instability. The argument here being that
even though without this prefactor there is a cusp in when one
electron moves through the ion, as the density close to the ion is so
low this is a very unlikely event.
The optimisations performed above were repeated for the with no
term in the prefactor.
The above
assumption proved to be correct in that this didn't appear to produce
any serious instability into the optimisation. However, it failed to
produce any significant further reduction in either the energy or
variance of energy of the configurations.
