In the following chapter, results are presented for calculations
performed on solid silicon in the diamond structure. The variance
minimisation process for silicon is very similar to that for germanium
except that a non-local pseudopotential is used in the silicon
calculations. The inclusion of a non-local pseudopotential
complicates the optimisation process somewhat because its expectation
value depends of the value of the wavefunction throughout all space.
This means that the value of the non-local potential energy cannot be
stored as a constant during the optimisation procedure in the same way
as the local potential energy. Any change in the values of the
variational parameters in the wavefunction will effect the value of
the non-local energy. This change then requires a full
3*N*-dimensional integral to be evaluated for each change in the
parameters. Several different techniques have been experimented with
to try and deal with the non-local pseudopotential.

- Keeping the Non-Local part Fixed during Optimisation
- Evaluating the non-local Integral during Optimisation

Tue Nov 19 17:11:34 GMT 1996