<p>The mechanisms of radiationless relaxations of cytosine and guanine in the Watson–Crick model through conical intersections S₀/S₁ were studied for the first time at the DFT level of theory (B3LYP/aug-cc-pVDZ) in water surroundings (PCM). The conical intersections S₀/S₁ found are connected with distortions of aromatic rings in the area of H-bonding between the two nucleobases. The mechanism of cytosine distortion shows that the driven state is the dark ¹nπ^* excited state, but not the ¹ππ^* excited state as known for single cytosine molecule. The distortion mechanisms connected with guanine monomer in the base pair occur along the ¹ππ^* excited-state reaction curves and lead to conical intersections S₀/S₁. The driven ¹ππ^* excited states have higher energies, whereas the lower ¹ππ^* excited states, which show energy increase along the reaction coordinate, have charge transfer state origin.</p> Graphical abstract <p></p>