Perplexing Fukui Indices about xtb

[haneug]

Otherwise they should be correct. They are calculated according to equations 2a, 2b, and 2c of this paper (also shown at the end of the abstract) where the term Mulliken charges can be misleading in my opinion since I believe what they are actually using is what I understand as Mulliken population. The difference with SCM I would attribute mostly to them using the Hirshfeld partitioning (probably a better idea) and secondly on the STOs they use instead of GTOs, although this difference I assume should be very small.

[corinwagen]

Note that similar issues have been seen before: #794

[corinwagen]

Similarly when I compute f(-) for fluorobenzene, I get ortho = -0.049, meta = -0.045, and para = -0.081.

The literature values are ortho = 0.047, meta = 0.026, and para = 0.093. Granted, these data are computed using HF/3-21G so it's not surprising that the numerical values are different, but the total flip in sign makes me think there's something pretty substantially wrong with the xTB values.

[haneug]

Indeed, the sign switch I introduced in #794 seems to be wrong since we are using Mulliken charges and not Mulliken populations. Therefore, the sign of the fukui functions is switched! I will submit a PR to fix this. Thank you for bringing this to our attention.

[corinwagen]

any time. the values are correct otherwise?