Abstract Using extended Hückel theory (EHT), a theoretical study of the preferred conformations of ATP and ADP are compared with the experimentally observed structures, as the second of a set of studies on the molecular conformations of AMP, ATP and ADP. Results show that EHT yields a minimum energy ATP configuration that corresponds to the observed structure when atoms from the crystal are included in the calculations. Eight torsional angles were examined for ATP and six for ADP with all atoms included in the calculations. Results indicate that torsional rotations involving the adenosine part of the molecule show well-defined local minima. The predominant feature of the pyrophosphate section, however, is a low energy profile enabling the molecule to adapt its conformation to the environmental conditions.