CASTEP supports one local exchange-correlation functional, LDA, three gradient-corrected exchange-correlation functionals, GGA, and a set of nonlocal functionals for the self-consistent total energy calculations.
CA-PZ is the only local (LDA) functional available in CASTEP. It is based on the Ceperley and Alder (1980) data as parameterized by Perdew and Zunger (1981).
GGA functionals provide a better overall description of the electronic subsystem than the LDA functionals. The LDA description tends to overbind atoms, so that the bond lengths and the cell volume are usually underestimated by a few percent and the bulk modulus is correspondingly overestimated. GGA corrects this error but may underbind instead, leading to slightly long bond lengths.
PBE (Perdew et al., 1996) is the default exchange-correlation functional. It is recommended, especially, for studies of molecules interacting with metal surfaces, although it is also fairly reliable for bulk calculations. The PW91 (Perdew et al., 1992) functional should be used for comparisons with literature data, as it is the most widely used GGA functional. The results obtained with the PW91 functional should be very similar to those obtained with PBE. Another nonlocal functional, RPBE (Hammer et al., 1999), is a revised form of the PBE functional designed to improve the description of metallic surfaces. One of the latest GGA potentials, WC, is due to Wu and Cohen (2006). The authors claim significant improvements for lattice constants, crystal structures, and metal surface energies over the most popular Perdew-Burke-Ernzerhof (PBE) GGA. Another recent functional, PBEsol, has been developed specifically to improve the description of exchange in solids, resulting in better structures and energetics for densely packed solids and their surfaces (Perdew et al., 2008).
Nonlocal exchange-correlation functionals result from the generalized Kohn-Sham procedure (Seidl et al., 1996) and are intended to improve on the description of band gaps in insulators and semiconductors compared with LDA or GGA calculations. This additional accuracy comes at the price of more time consuming calculations.
Several approximations for nonlocal exchange-correlation functionals are available:
The recommended functionals are sX-LDA, PBE0, B3LYP and HSE06; the rest are supported mostly for testing against other packages that implement Hartree-Fock schemes for solids.
There are limitations on the use of this approach: nonlocal exchange can only be used for insulators using the all-bands minimizer (not density mixing) with the Energy, Geometry Optimization, and TS Search tasks (without cell optimization). It is not compatible with stress, NMR, phonon or polarizability calculations. The formalism cannot be used with mixture atoms or ultrasoft potentials. Note that stress calculations, and hence cell optimization and the Elastic Constants task, are supported for screened exchange versions, sX and sX-LDA, and for the HSE family of functionals.
To select the exchange-correlation functional
CASTEP background theory
Setting up electronic options
Setup tab - CASTEP Calculation dialog