AEXX
AEXX = [real]
| Default: AEXX | = 0.25 | if LHFCALC=.TRUE. |
| = 0 | otherwise |
Description: AEXX specifies the fraction of exact exchange in a Hartree-Fock-type/hybrid-functional calculation.
| Important: The sum of the fraction of the exact exchange and local-density-approximation (LDA) exchange is always 1.0, i.e., it is not possible to independently set the amount of LDA exchange. |
For instance, if AEXX=0.25, [math]\displaystyle{ 1/4 }[/math] of the exact exchange is used and [math]\displaystyle{ 3/4 }[/math] of the LDA exchange is added. For AEXX=0.5, half of the exact exchange is used, and one-half of the LDA exchange is added. In contrast, for semi-local generalized-gradient approximation (GGA), the amount of GGA exchange (AGGAX) and GGA correlation (AGGAC) can be set independently. In fact, some popular hybrid functionals use for instance only 0.8 of the gradient contribution to the exchange. The tags AGGAX and AGGAC are only used if GGA is already selected. For LDA calculations, no gradient correction will be added regardless of the value set for AGGAX and AGGAC.
| Mind: The defaults are chosen such that the PBE0 functional is selected. |
That is, the PBE0 functional contains 25% of the exact exchange, and 75% of the PBE exchange, and 100% of the PBE correlation energy. The resulting expression for the exchange-correlation energy then takes the following form:
[math]\displaystyle{ E_{\mathrm{xc}}^{\mathrm{PBE0}}=\frac{1}{4}~E_{\mathrm{x}}~+ ~\frac{3}{4}~ E_{\mathrm{x}}^{\mathrm{PBE}} +~E_{\mathrm{c}}^{\mathrm{PBE}} }[/math]
A comprehensive evaluation of the performance of the PBE0 functional, as compared to PBE, can be found in Ref. [1].
For AEXX=1.0, VASP automatically sets ALDAC=0.0 and AGGAC=0.0, and thus runs a full Hartree-Fock-type calculation.
Related Tags and Sections
AGGAX, AGGAC, ALDAC, LHFCALC, Hybrid functionals, List of hybrid functionals