Geometry Optimisation with CASTEP

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Checking the absolute convergence of
our Bulk Silicon Calculation
The example calculation with bulk silicon does not produce a very accurate answer. Can it be improved?
There are two main sources of error in our calculation;
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Fundamental errors associated with the approximations used in the calculation such as the use of
the LDA.
Systematic errors associated with inaccuracies in the wavefunctions introduced by using a finite
basis set and wavefunction sampling in reciprocal space.
The latter of these must be reduced to a minimum. To show how this might be done I have repeated the
calculation several times with increasing precision in the basis set and with various MP grid densities.
Figure 3 below shows the total energy as a function of the basis set size for calculations with 1, 10, 28
and 60 K-Points. (See the section on variable cell calculations for a neat trick to efficiently perform many
singlepoint energy calculations at a variety of cutoff energies at a fixed k-point density).