Floquet Time-Dependent Configuration Interaction for Modeling Ultrafast Electron Dynamics

Time-dependent electronic structure methods are a valuable tool for simulating spectroscopic experiments. Recent advances in time-dependent configuration interaction (TDCI) algorithms have made them an attractive means of modeling many-electron dynamics, particularly for cases where multireference effects are essential. Here we present an extension to TDCI, Floquet TDCI, where the electronic wave function is expanded in a basis of light-dressed determinants. Our approach is based on our high-performance graphics processing unit (GPU) accelerated implementation of complete active space configuration interaction (CASCI). Simulations of two-photon absorption demonstrate that Floquet TDCI is well-suited for modeling dynamics in intense, ultrashort laser pulses. Accurate results are obtained for pulse energies up to ∼4 × 10^-4 J/cm² per pulse in the most difficult case explored here. By simulation of a set of molecules under continuous wave coupling, we demonstrate the ability of Floquet to describe the entanglement of light and multiple molecules in a cavity (i.e., a cavity polariton). Excellent computational performance is observed: a 320 fs propagation of a large dye (C₃₀N₂H₂₂) with a 2 as timestep and a large active space (10 electrons in 11 orbitals), including a monochromatic pulse with three photon states, was performed in 3 h 6 min on a single Tesla V100 GPU. Our Floquet TDCI algorithm scales linearly with the number of photon states and exponentially with the number of photon colors included in the calculation. We argue that its energy-conserving nature makes Floquet TDCI well-suited to drive nonadiabatic molecular dynamics simulations.