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Trial Wavefunction

In the following calculations it has been assumed that the addition (removal) of a single electron does not significantly affect the overall shape of the exchange correlation hole and so the same Jastrow correlation function that was introduced in chapter gif has again been used. It was also assumed that although the addition(removal) of a single electron will reduce the symmetry of the charge density, this will not be a strong effect. The same form of the 1-body function, grouped into stars according to the higher point group symmetry of the crystal in its ground state as described in section gif, should therefore still be applicable. The validity of these approximations in the trial wavefunction is tested in section gif, where the calculations are repeated in DMC. The value of the energy calculated in DMC is independent of the quality of the u and parts of the trial wavefunction. Therefore, if the quality of the u and parts of the trial wavefunction is much lower for the calculations where an electron has been added or removed, one would expect to see a bigger reduction in the energy when moving from VMC to DMC.

The trial wavefunctions for each of the tex2html_wrap_inline7975 , tex2html_wrap_inline7977 and tex2html_wrap_inline7979 systems were individually optimised using the variance minimisation techniques described in chapter gif. Six stars of tex2html_wrap_inline7079 vectors were used to describe the functions and 22 parameters were used to describe the u functions, as in chapter gif. Large ensembles of up to 1 million electron configurations were used in each iteration of the optimisation procedure and 3-5 iterations were performed to obtain ground state wavefunctions with an accuracy of approximately tex2html_wrap_inline7561  0.01eV per atom within the parameter space of the optimisation. This is equivalent to an accuracy of approximately tex2html_wrap_inline7561  0.3eV in the energy of the gap as given by Eq.(gif).


next up previous contents
Next: Electron-Electron Interaction Up: Addition and Subtraction of Previous: Addition and Subtraction of

Andrew Williamson
Tue Nov 19 17:11:34 GMT 1996