Surface Hopping with Fully Correlated Methods

Surface hopping simulations critically depend on the accuracy and robustness of the underlying electronic structure methods. Fully correlated approaches─such as CASPT2, MRCI, L-PDFT, and MRSF-TDDFT─that account for both dynamic and static electron correlation (without implying an exact treatment of electron correlation) offer significant promise. Still, their practical application in dynamics remains limited by the computational cost and technical challenges. In this Perspective, we examine the current status of such methods by analyzing representative surface-hopping simulations of fulvene and pyrrole, two prototypical systems for photophysical and photochemical processes. These examples demonstrate that while fully correlated methods improve the description of bond rearrangements and hot ground-state dynamics, partially correlated approaches─such as ADC(2) and TDDFT─remain sufficient for photophysical excited-state relaxation. Across methods, persistent limitations, such as active-space instabilities and potential-energy discontinuities, imply the need for improved approaches. We argue that expanding the use of generalized active spaces in the short term and advancing large active space algorithms in the long term will be crucial for making high-accuracy nonadiabatic simulations broadly reliable and accessible.