Hartree-Fock and hybrid potentials

Hartree-Fock and the hybrid potentials can not or should not be used in combination with geometry optimization, TS, IRC, LT, frequencies, NMR chemical shift, spin-spin coupling, time dependent DFT, EPR g-tensor, frozen cores.

Numerical problems have been found with the present implementation of Hartree-Fock or hybrids during the SCF, especially if the molecule has symmetry NOSYM and a basis set TZP or larger is used. Workaround is to use always the DEPENDENCY key with rather strict criteria for the basis set dependence, namely bas=4e-3. In the ADF2006.01 the DEPENDENCY key is automatically switched on in the case of a Hartree-Fock or a hybrid potential. The result of the DEPENDENCY key is that linear dependence of the basis set is reduced by removing linear combinations that correspond with eigenvalues in the virtual SFOs overlap matrix, which are smaller than, in this case, 4e-3. Note that this is a rather large value, such that it will have an effect on the bonding energy. For DZ and DZP basis sets this value will normally not result in reduction of the virtual space. However, for TZP, TZ2P, QZ4P and larger this will often result in reduction of the basis set, which will have an effect on the accuracy of the bonding energy. In these cases one could try a smaller value than 4e-3, but be aware that numerical problems may occur. If the molecule has symmetry the numerical problems are reduced.

The origin of this problem is that for an accurate description of Hartree-Fock exchange one needs more (diffuse) fit functions in the fit procedure which is used in ADF, which uses only fit fuctions on the two centers of the two STOs. One can get more diffuse fit functions if one adds in the Create run of an atom the key:

AddDiffuseFit

If the BASIS key is used one can also add this key in the molecular calculation (the scripts in ADF will then automatically add this in the Create runs of the atoms). If one adds this key preliminary results indicate that one can lower the value for the dependency key to bas=1e-4. Such a low value for the dependency key normally means that the basis set is not reduced for basis sets of TZP or TZ2P quality.