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1、Setting up SCF parametersSCF 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 opti
2、on 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 Electronic Options dialog, need not be changed und
3、er 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 ).Note. The CASTEP server automatically increases the number specified in the interface by a fact
4、or of three for metallic systems.Electronic minimization algorithmThe algorithm that is used to solve the DFT equations is specified by the Electronic minimizer option on the SCF tab on the Electronic Options dialog. Density mixing is the recommended choice, in terms of both robustness and efficienc
5、y. We found it to be 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 de
6、nsity 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
7、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.Variable electronic states occupanciesBy default CASTEP uses variable electronic occupancies, thus effective
8、ly 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 sufficientlylarge to cater for nearly degenerate bands close to the Fermi level. This is relevant for transition or rare e
9、arth 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 lik
10、ely to be noticeably nonzero for at least some k-points.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)
11、offers a more robust alternative.Tip. 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.Further informationSetting up electronic optionsElectronic - CASTEP CalculationSCF - Electronic OptionsDensity Mixing Options - SCF
