In parallel, advances in computational power enabled the development of methods to simulate the nonadiabatic wavepacket dynamics underlying the femtosecond time-dependent experimental observables . Such studies are challenging as they require an accurate treatment of multiple electronic states, including e.g., the potential energy surfaces of excited states and the nonadiabatic coupling between two electronic states. Recent advances in computer hardware and algorithms enable significant improvement in the efficiency and accuracy of molecular simulations. In particular, our earlier success in leveraging graphical processing units (GPUs) for fast electronic structure calculations (51-55) enables on-the-fly, nonadiabatic, excited state dynamics simulations using GPU-accelerated electronic structure methods, such as LR-TDDFT , CASCI , FOMO-CASCI (58; 59), SA-CASSCF (60; 61), α-CASSSCF (17), CASPT2 (62; 63), and EOM-CCSD .