next up previous contents
Next: Optimising Wavefunctions for Atoms Up: Optimising Trial Wavefunctions Previous: Tests on Jellium

Applying the New u Function to Solids

Having studied the HEG, the newly developed u function was applied to a crystalline solid. To enable direct comparison with the previous results, germanium in the diamond structure was used as a test material. The same fcc simulation cell of diamond structure germanium containing 16 atoms was studied. The same single-particle orbitals were used to construct the Slater determinant. The tex2html_wrap_inline6237 function was chosen to have the full symmetry of the diamond structure. Again the tex2html_wrap_inline6237 function was expanded in a Fourier series, grouping the tex2html_wrap_inline7079 vectors into stars as in Eq.(gif).

For the u function, the functional form of Eqs.(gif-gif) which was developed for the HEG was chosen. The u and tex2html_wrap_inline6237 functions were optimised simultaneously because they are strongly coupled. Typically 6 non-zero coefficients in Eq.(gif) for the tex2html_wrap_inline6237 function and 8 parameters for both the parallel- and antiparallel-spin u functions in Eq.(gif) were used, giving a total of 22 parameters in the minimisation problem. Variance minimisations were carried out using 10,000-100,000 independent N-electron configurations, which were regenerated several times. The final energy of -107.69 tex2html_wrap_inline7561 0.01 eV per atom is 0.08 eV lower than the result obtained using the (Ewald summed) Yukawa potential of Eq.(gif) and the variance minimisation procedure for tex2html_wrap_inline6237 , and 0.20 eV lower than the result obtained in our previous work using the Yukawa potential and Fahy's original prescription for tex2html_wrap_inline6237 [48, 26]. The energy of -107.69 eV per atom is only 0.34 eV per atom higher than the DMC result for this system of -108.03 tex2html_wrap_inline7561 0.07 eV per atom quoted in Table I of Ref. [50]. (As discussed in Refs. [33, 50], we estimate that about 0.12 eV of this energy difference is due to the basis set incompleteness error in the single-particle orbitals, which affects the VMC much more than the DMC result, and which could be eliminated by the use of a larger basis set or a smoother pseudopotential.)

The optimised spin-parallel and spin-antiparallel u functions for germanium are similar to the Yukawa form in all directions. However, they have a smaller derivative at intermediate distances, exactly as observed in the HEG (see figure gif). The optimised tex2html_wrap_inline6237 function differs significantly from the original Fahy form, with some parameters changing by an order of magnitude. Altering the number of parameters in the optimisation scheme revealed that 6 non-zero coefficients was again sufficient to converge the tex2html_wrap_inline6237 function.


next up previous contents
Next: Optimising Wavefunctions for Atoms Up: Optimising Trial Wavefunctions Previous: Tests on Jellium

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