Excited-state geometry optimizations. Excited-state geometry optimizations and SCS-ADC(2) calculations of potential energy profiles connecting the key stationary points and state crossings provide further insights into the photodeactivation mechanisms that can be attributed to the measured excited-state lifetimes of 2,6-dAP. In particular, the adiabatic excitation energies of the S 1 states (energy difference between the S 1 and S 0 minimum energy geometries) of the 7H and 9H tautomers are in good agreement with the measured origins of absorption. The calculated adiabatic excitation energy of the S 1 state of 7H 2,6-dAP amounts to 4.04 eV, which is in semi-quantitative agreement with the measured origin of absorption of 3.99 eV. In the case of the 9H tautomer the corresponding theoretical and experimental values amount to 4.62 eV and 4.33 eV, respectively. Though the energy difference is larger, the theoretical result is well within the error bars of the ADC(2) method ( ) and the qualitative trend is retained, in the sense that 9H 2,6-dAP has clearly higher adiabatic excitation energy of the S 1 state. Using the S 1 minimum energy geometries and different minimum-energy crossing points found at the SCS-ADC(2)/SCS-MP2 level of theory, we constructed several potential energy profiles which can be assigned to different excited-state lifetimes derived from our pump-probe measurements, discussed below.