Information on the Atoms In Molecules (AIM) scheme can be found on the following sites:
The AIM version in Gaussian can be used with the aim keyword. The following input file will perform a minimal AIM analysis in order to obtain AIM atomic charges:
#P HF/STO-3G scf=(direct,tight) aim=charges test29 HF/STO-3G formaldehyde AIM 0 1 C1 O2 1 r2 H3 1 r3 2 a3 H4 1 r3 2 a3 3 180. r2=1.21672 r3=1.10138 a3=122.73682
A full AIM analysis can be performed with aim=all.
A detailed overview of the performance of the AIM method in describing the charge distribution in water can be found in: F. Martin, H. Zipse, J. Comp. Chem. 2005, 26, 97 - 105. A copy of the pdf file is available here.