These images are constructed by contouring the difference density, , constructed by subtracting from the total electron density function t he sum of the electron densities of the isolated atoms, placed at the appropriate position in the molecule.

So,

Note that the integral of is equal to the total number of electrons in the molecule

and the integral of or is equal to the total number of electrons in atoms A or B.

Note that in a neutral molecule

will be positive or negative. The regions of space where is negative are where electrons have moved from in the process of forming the chemical bond. The positive regi ons are where electrons have moved into. is constructed from accurate molecular wavefunctions and & from accurate atomic wavefunctions.