SCF settings determine the algorithm that CASTEP uses to find the ground state of the electronic subsystem, as well as the accuracy required. Most of these settings do not need to be changed by the user. For example, the SCF tolerance is controlled by the global Quality option on the Setup tab. It can also be modified using the SCF tolerance option on the Electronic tab, however this is not recommended.
Similarly, the maximum number of SCF cycles, which can be adjusted using Max. SCF cycles option on the SCF tab on the CASTEP Electronic Options dialog, need not be changed under normal circumstances.
The Max. SCF cycles setting determines how many SCF steps are taken by CASTEP before it moves atoms according to the task being performed (i.e. Geometry Optimization or Dynamics).
The CASTEP server automatically increases the number specified in the interface by a factor of three for metallic systems.
The algorithm that is used to solve the DFT equations is specified by the Electronic minimizer option on the SCF tab on the CASTEP Electronic Options dialog.
Density mixing is the recommended choice, in terms of both robustness and efficiency. It is 2-4 times faster for insulators than the conjugate-gradient based All Bands/EDFT scheme. The Density mixing scheme is especially good for metallic systems, where speedups for metal surfaces compared to conjugate gradient schemes are in the region of 10-20.
The only case where density mixing may not improve performance is for molecule in a box calculations.
The default density mixing settings use Pulay mixing and conjugate-gradient minimization of each electronic state. You should only attempt to change these parameters if SCF convergence is very poor. Sometimes it helps to reduce the length of the DIIS history from the default value of 20 to a smaller value (5-7). It might also be helpful to decrease the mixing amplitude from the default value of 0.5 to 0.1-0.2.
By default CASTEP uses variable electronic occupancies, thus effectively treating all systems as metallic. This is recommended, as it speeds up density mixing optimization, even for systems with large band gaps. The number of empty bands should be sufficiently large to cater for nearly degenerate bands close to the Fermi level. This is relevant for transition or rare earth metal compounds, where narrow d or f bands can be pinned at the Fermi level.
If the number of bands used for such a system is insufficient, the SCF convergence will be very slow and probably oscillatory. Occupation
numbers of the highest electronic states as reported in the
.castep file are likely to be noticeably nonzero for at least some
SCF convergence with the Density mixing minimization scheme can sometimes be poor for metallic systems. If this is the case, the alternative All Bands/EDFT scheme, which is based on the ensemble density-functional theory (Marzari et al., 1997) offers a more robust alternative.
Slow SCF convergence is often indicative of an insufficient number of empty bands, especially in spin-polarized calculations. To check if this is the cause, inspect the occupancies of the highest energy electronic states. They should be very close to zero for all k-points in a calculation which is setup correctly.
By default CASTEP does not perform dipole corrections, however, for slab and single molecule systems with P1 symmetry either a Self-consistent or Non self-consistent dipole correction can be applied. The self-consistent scheme is based on the method suggested by Neugebauer and Scheffler (1992) where the correction potential is recalculated at each SCF step. The non self-consistent scheme is based on the approach of Yeh and Berkowitz (1999) where a dipole correction is added after the SCF has converged. In this scenario only the total energy and its gradients are corrected, not the electrostatic potential. As a result, this type of correction is not suitable for electrostatic potential studies such as workfunction calculations.
Dipole corrections can be essential in eliminating nonphysical electrostatic interactions between periodic images, improving accuracy of calculated adsorption energies for molecules on surfaces, for example.
Setting up electronic options
Electronic tab - CASTEP Calculation dialog
SCF tab - CASTEP Electronic Options dialog
CASTEP Density Mixing Options dialog