Complete Active Space Self-Consistent Field with GPU-Accelerated Density Fitting

The complete active space self-consistent field (CASSCF) method is essential for describing complex photochemical processes, but its application in ab initio molecular dynamics is often limited by the computational cost associated with four-center two-electron repulsion integrals (ERIs). We present the first implementation of a GPU-accelerated density fitting (DF) approximation for CASSCF, developed within the TeraChem software package. Validation on salicylaldimine demonstrates that the DF approximation introduces negligible errors in relative energies, yielding excitation energies accurate to within 10 microHartrees of the integral-direct reference. The DF-CASSCF implementation achieves significant computational speedups, accelerating total energy and gradient calculations by more than an order of magnitude for small- to medium-sized systems with large atomic orbital basis sets. We demonstrate the practical utility of this approach through ab initio multiple spawning dynamics simulations of excited-state intramolecular proton transfer (ESIPT). The DF-CASSCF trajectories reproduce the photodynamics of the reference simulations while reducing the total wall time (for a single GPU) by a factor of 3 to 30 depending on the choice of basis set. This work significantly lowers the barrier for high-throughput, high-accuracy multireference simulations on modern GPU architectures.