Geometry Optimisation with CASTEP

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Variable Cell Calculations
A variable cell calculation is performed if we are attempting to geometry optimise a material for which
the lattice parameters and atomic positions are not even approximately known. In this situation, placing
constraints on the size and shape of the supercell might affect the result. A ground state structure with a
large lattice constant might not be found if a small supercell was used which confined the atoms to be too
close together.
The solution is to perform a variable cell calculation which will allow the supercell size and shape to be
optimised along with the atomic positions. A variable cell calculation is the default and will be
performed as long as FIX_ALL_CELL : TRUE is not in the .cell file. A finite basis set correction [3]
is used to reduce errors associated with changes in the total number of plane waves as the system changes
size. As the supercell changes size the basis set associated with each k-point is altered. To adjust for this
the cutoff energy could be varied to maintain the number of plane waves as a constant. However, it is
generally considered [4] to be more acceptable to keep the cutoff energy constant and vary the number of
plane waves. This 'graininess' in the basis set can be smoothed out with a finite basis set correction. This
involves the calculation of the variation of the total energy with the logarithm of the cutoff energy, which
is used as a smoothing parameter.
The principle keyword that activates a finite basis set correction whenever the cell parameters change is
finite_basis_corr : n
The default value of n is none, meaning no finite basis set correction is performed. The user can specify
the value of the smoothing parameter by setting n = manual and then including
basis_de_dloge: v
where v is the value of the smoothing parameter.
There is also the handy option of allowing castep to automatically perform a finite basis set correction.
Set
finite_basis_corr : auto
The correction is made by performing several total energy calculations for a given configuration of atoms
but using different cutoff energies in each one. The variation in the total energy with the logarithm of the
cutoff energy extrapolated from these singlepoint calculations enables the evaluation of the smoothing
parameter. The user can specify.