CASTEP allows you to perform calculations on charged systems. This is useful for studies of, for example, charged defects in semiconductors (Milman et al., 1993 and Hakala et al., 2000).
Makov-Payne corrections, which improve the convergence of the total energy with respect to the supercell size in charged periodic systems, are not implemented in this version of CASTEP. This means that the results obtained for charged systems with small unit cells are less reliable.
To set system charge
Magnetic systems can be studied by carrying out spin-polarized DFT calculations with CASTEP. This setting is recommended for transition metal oxides, certain inorganic surface studies, and metallic systems containing magnetic elements (Fe, Co, Mn, Ni).
Spin-polarized CASTEP calculations are normally carried out with a variable magnetic moment. This procedure, however, does not guarantee finding the lowest energy state. DFT solutions can converge to a variety of local minima, so that the metastable states are found. The most commonly obtained solutions correspond to high spin and low spin states. The state generated by CASTEP depends on the initial magnetic configuration, since the solution is likely to converge to the nearest local minimum rather than to the global minimum.
There are two ways of defining the initial magnetic configuration: either specify the total magnetic moment per unit cell, which gets uniformly distributed over the space, or provide detailed information on the absolute values and direction (up or down) of the spins for each atom in the unit cell. The former method can be used for relatively simple systems where only two solutions are expected (magnetic and non-magnetic). The latter method, which specifies the spin state of the atoms in the system, is more general and gives much more flexibility. It is possible to set up ferromagnetic, ferrimagnetic, or antiferromagnetic calculations to get different starting spin arrangements.
To set up a spin-polarized calculation with a uniformly distributed initial magnetic moment
To set up a spin-polarized calculation with a specified ordering of atomic moments
Initial magnetic moments on atoms are treated correctly by CASTEP only when the density mixing scheme is used for electronic structure minimization. CASTEP will ignore atomic information and will use the uniformly distributed magnetic moment for the all bands or EDFT minimizers. The value of the total moment is determined by the sum of formal spins in the system.
It is important to include a sufficient number of empty bands when optimizing the spin state. The default number of empty bands is 4, which means that the maximum accessible increase in the spin value is 8. A calculation that started with the low spin state will be extremely slow if the actual minimum found by CASTEP corresponds to the high spin state (although this is not the case when a calculation starts with high spin and converges to low spin).
CASTEP might fail to converge the SCF if the number of empty bands is insufficient to accommodate the high spin state, especially in a calculation with variable occupation numbers (metallic systems).
An external electric field can be applied to the system. This causes a force to act on each atom which is proportional to the charge on that atom. This can be used to calculate the polarizability of a dielectric material.
To apply an electric field
Setting up electronic options
Setup tab - CASTEP Calculation dialog
SCF - Electronic Options