Calculated electron densities from PBE0/6-31+G(d,p) were analyzed with respect to the hydrogen bonding within a nucleic acid base pair and the pi-stacking between sets of base pairs. From published X-ray crystallographic data, base pairs were isolated from a total of 11 DNA and RNA duplexes, and their experimental geometry was maintained throughout the analyses. Focusing solely on Watson Crick base pairs, from the values of the electron density between interacting nuclei (at the bond critical points), we provide quantitative data on individual weak interactions. For hydrogen bonding, in addition to quantifying the scissoring effect in GC base pairs, the origin of the controversy around the relative stability of AT and AU base pairs is identified and resolved. Thus, it is illustrated how the conclusion as to their relative stability rests on the specific choice of oligonucleotides compared. For pi- stacking, sequence effects for tandem AT base pairs are captured, quantified, and explained, and the greater sensitivity of GC, over AT, sequences to the rise parameter is established. The results presented here show that, from experimental geometries and their electron densities, previously determined effects of the sequence and structure of a duplex on the stabilizing interactions can be captured, quantified, and traced back to the geometry of the base pairs.