Excited-state molecular dynamics is essential to the study of photochemical reactions, which occur under nonequilibrium conditions. However, the computational cost of such simulations has often dictated compromises between accuracy and efficiency. The need for an accurate description of both the molecular electronic structure and nuclear dynamics has historically stymied the simulation of medium- to large-size molecular systems. Here, we show how to alleviate this problem by combining ab initio multiple spawning (AIMS) for the nuclear dynamics and GPU-accelerated state-averaged complete active space self-consistent field (SA-CASSCF) for the electronic structure. We demonstrate the new approach by first-principles SA-CASSCF/AIMS nonadiabatic dynamics simulation of photoinduced electrocyclic ring-opening in the 51-atom provitamin D3 molecule.