Or with thicker sticks:
In fact there is of course not so much space between the bases. Here is what it looks like when you draw the van der Waals spheres for the bases. It is actually quite difficult to model this stacking interaction because it is almost pure dispersion. From a computational point of view, dispersion is electron correlation. If the effect you are interested in is pure electron correlation, then you'd better model it really well. Hartree-Fock gives you zero dispersion. MP2 is great if you kind of want to include some electron correlation but it is not accurate enough in this case. What they are actually doing is high level coupled cluster CCSD(T) extrapolated to the complete basis set limit.
Another interesting question is how defects are propagated in such a framework: holes, electrons, or electron hole pairs. First of course you want to preserve the integrity of your genome. And second you want to make nano-robots. DNA is already a self-assembling structure with molecular recognition. If charge transport is better understood and conductive analogues are found, DNA will kick nanotubes' ass.
By the way: for printing out pymol graphics (see again ), the ray_trace_mode setting is nice. Especially:
Then you get some nice black frames instead of fuzzi ends in the print.
 from the crystal structure 1BNA. And drawn with pymol. Maybe I should eventually switch from pymol because they are becoming more and more commercial. But then I guess they would not mind me as a little personal user.