It is shown that the total charge density is a valid source to confirm hydrogen bonding without invoking a reference charge density. A set of criteria are proposed based on the theory of "atoms in molecules" to establish hydrogen bonding, even for multiple interactions involving C-H ... O hydrogen bonds. These criteria are applied to several van der Waals complexes. Finally a bifurcated intramolecular C-H ... O hydrogen bond is predicted in the anti-AIDS drug AZT, which may highlight a crucial feature of the biological activity of a whole class of anti-AIDS drugs.