A common question in organic chemistry is that of regioselectivity: “where will my molecule react?” This is an important question, but it can be difficult to predict (automated transition state finding is unreliable, and even if it were reliable, it would quickly become quite expensive).
One of the more interesting solutions to this problem is “Fukui indices,” named after Kenichi Fukui (1981 Nobel Laureate in Chemistry), which quantify how a molecule's electron distribution changes upon adding or removing an electron.
To compute atom-centered (“condensed”) Fukui indices, Rowan computes and takes the difference between the atomic charges of the neutral molecule and the corresponding anion or cation.
Each atom has three different Fukui indices:
(f(0) is computed by taking the difference between the atomic charges of the anion and the cation.)