2,6-diaminopurine based on MS-CASPT2/SA-CASSCF geometry optimizations and calculations of excited-state potential energy profiles. In particular, the authors located C2-puckered and C6-puckered S 1 /S 0 state crossings for both tautomers, which are analogous to the C2-and C6-puckered S 1 /S 0 state crossings discussed in this work. The MS-CASPT2/SA-CASSCF approach should generally offer a more accurate description of excited-state potential energy surfaces and S 1 /S 0 state crossings, particularly in the regions where the ground-state wave-function gains a highly multi-configurational character.