Setting the exchange-correlation functional

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.

LDA functional

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

GGA functionals provide a better overall description of the electronic subsystem than the LDA functionals. The LDA description tends to overbind atoms, causing underestimation of the bond lengths and the cell volume by a few percent and overestimation of the bulk modulus. 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. PBE is especially suitable for studies of molecules interacting with metal surfaces, although it is also fairly reliable for bulk calculations. Use the PW91 (Perdew et al., 1992) functional for comparisons with literature data, as it is the most widely used GGA functional. The results obtained with the PW91 functional are usually 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, was developed by 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, was 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).

Meta-GGA functional

In addition to the generalized gradient (GGA) functionals, which depend on the local density and its gradient, CASTEP can handle functionals that depend on the kinetic energy density. The current supported functional is RSCAN (Bartok and Yates, 2019), which is an improved regularized version of the SCAN ("Strongly Constrained and Appropriately Normed") functional developed by Sun et al., 2015. Meta-GGA functionals are considered to be more accurate than pure GGA, and the cost of such calculations is significantly lower than for nonlocal functionals.

There are limitations on the use of meta-GGA in CASTEP. The meta-GGA functional is not compatible with spin-orbit coupling, J-coupling, linear-response phonons, or polarizability calculations. You cannot use the formalism with mixture atoms. You can only use on-the-fly-generated pseudopotentials.

Nonlocal exchange-correlation functionals

Nonlocal exchange-correlation functionals result from the generalized Kohn-Sham procedure (Seidl et al., 1996). The intention is 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 preferred functionals are sX-LDA, PBE0, B3LYP, and HSE06. The other functionals allow testing against other packages that implement Hartree-Fock schemes for solids.

There are limitations on the use of this approach: you can only use nonlocal exchange for insulators using the all-bands minimizer (not density mixing). It is not compatible with NMR, phonon, or polarizability calculations. You cannot use the formalism with mixture atoms or ultrasoft potentials.

You can customize nonlocal functional by specifying explicitly which LDA or GGA functional expressions to combine with which nonlocal functional, and with what weights. You can achieve this by adding a block XC_DEFINITION to the .param input file. For detailed steps for editing input file using Save Files and Run Files, see Manipulating Files.

To select the exchange-correlation functional

  1. Choose Modules | CASTEP | Calculation from the menu bar.
  2. Select the Setup tab.
  3. Choose a local (LDA), a gradient corrected (GGA) or nonlocal functional from the first dropdown list for the Functional.
  4. Select the specific functional from the second dropdown list.
See Also:

CASTEP background theory
Setting up electronic options
Setup tab - CASTEP Calculation dialog