electronic coupling between adjacent bases, belonging to the same and to complementary stacks of DNA, is calculated using an ab initio procedure at the HF level. The mutual geometry of pair-bases has been accurately selected from the available crystallographic data and the variance of the coupling with the geometry is discussed. Results are applied to the description of single step hole transfer between two guanines separated by a base bridge. The effects of interstrand jumps and bases sequence on the hole transfer mechanism are considered.