Defaults and special cases

If the XC key is not used, the program will apply only the Local Density Approximation (no GGA terms). The chosen LDA form is then VWN.

If only a GGA part is specified, omitting the LDA sub key, the LDA part defaults to VWN, except when the LYP correlation correction is used: in that case the LDA default is Xonly: pure exchange.

The reason for this is that the LYP formulas assume the pure-exchange LDA form, while for instance the Perdew-86 correlation correction is a correction to a correlated LDA form. The precise form of this correlated LDA form assumed in the Perdew-86 correlation correction is not available as an option in ADF but the VWN formulas are fairly close to it.

Be aware that typing only the sub key lda, without an argument, will activate the VWN form (also if LYP is specified in the GGA part).

The LB94, GRAC, and SAOP functionals have only a SCF (=Potential) implementation, but no Energy counterpart. Therefore, they must not be used together with the Energy specification for Apply. If LB94 or GRAC is used for the Potential (SCF), the gga energy expression defaults to Becke (exchange part) + Perdew (correlation). For SAOP, the energy functional is PW91. This can be overruled by selecting another choice in the 'gga Energy ...' specification. However, it is recommendable to use a GGA for the xc potential if the main interest is in energies.

The LB94, GRAC, and SAOP forms are density functionals specifically designed to get the correct asymptotic behavior. This yields much better energies for the highest occupied molecular orbital (HOMO) and better excitation energies in a calculation of response properties (Time Dependent DFT). Energies for lower lying orbitals (sub-valence) should improve as well (in case of GRAC and SAOP, but not LB94). The energy expression underlying the LB94 functional is very inaccurate. This does not affect the response properties but it does imply that the energy and its derivatives (gradients) should not be used because lb94-optimized geometries will be wrong, see for instance [34]. The application of the LB94 functional in a runtype that involves the computation of energy gradients is disabled in ADF. You can override this internal check with the key allow.

In case of a GRACLB calculation, the user should be aware that the potential in the outer region is shifted up with respect to the usual level. In other words, the xc potential does not tend to zero in the outer region in this case. The size of the shift is the difference between the HOMO orbital energy and the IP given as input. In order to compare to regular GGA orbital energies, it is advisable to subtract this amount from all orbital energies. Of course, orbital energy differences, which enter excitation energies, are not affected by this shift in the potential.

The LB94, SAOP, and GRAC potentials cannot be used in a Create run (due to an implementation limitation in the code). If you need the energy difference of a molecule with respect to LB94-atoms, you have to run the single-atom calculations with LB94 separately, using the same non-LB94 Create atoms as fragments as you did for the whole molecule. This will give you the required energy corrections.