Robustness and reliability of different CASPT2 flavors for nonadiabatic molecular dynamics

Nonadiabatic molecular dynamics (NAMD) simulations are highly sensitive to the underlying electronic structure description. Within the complete active space second-order perturbation theory (CASPT2) framework, several multistate formulations, namely, multistate (MS)-, extended multistate (XMS)-, and rotated multistate (RMS)-CASPT2, are available, yet their relative robustness for on-the-fly NAMD remains an open question. In this work, we systematically assess the sensitivity of these three CASPT2 variants with respect to state averaging by performing NAMD simulations on two representative molecular test systems, fulvene and ethylene, using both trajectory surface hopping and ab initio multiple spawning approaches. For fulvene, pronounced differences are observed between the CASPT2 flavors. MS-CASPT2 shows a strong dependence on the number of averaged states, affecting not only excited-state lifetimes but also the nuclear evolution along key reaction coordinates. XMS-CASPT2 displays improved stability, with only minor variations in population dynamics and structural evolution upon increasing the number of states. RMS-CASPT2 proves to be the most robust formulation for this system, yielding consistent results across different state-averaging schemes. In contrast, ethylene represents a well-behaved case where all three CASPT2 formulations provide consistent dynamical results largely independent of state averaging. Overall, this work confirms the general reliability of CASPT2-based NAMD while highlighting that the choice of CASPT2 flavor and number of states can significantly influence both population dynamics and nuclear motion. Careful preliminary electronic-structure analysis and chemical intuition remain essential for meaningful CASPT2-based dynamics simulations. The benchmarking protocol presented here provides a foundation for future systematic assessments on more complex photochemical processes, aiming toward community-wide guidelines for the application of CASPT2 in nonadiabatic dynamics.