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We present a mixed quantum-classical simulation of the internal conversion between the lowest energy ππ* (SLa) and nπ* (Sn) states in adenine in gas phase, adopting a quadratic vibronic model (QVC), parametrized with the help of PBE0 Density Functional calculations. Our approach is based on a hierarchical representation of the QVC Hamiltonian and a subsequent treatment of the most relevant coordinates at accurate time-dependent quantum level and of the other 'bath' modes at classical level. We predict an ultrafast transfer (∽ 30 fs) of ∽75\% of the initial population excited on SLa to Sn. Within an adiabatic picture, on the same timescale the wave packet concentrates almost completely on the lowest S1 state, where however it shows a very broad distribution with different characteristics (due to the different 'diabatic' character). It is shown that the proposed methodology offers a practicable route to describe the quantum dynamics of internal conversion processes in large semi-rigid systems.